At the Wyllyotts Theatre, handily situated just round the corner from Potters Bar train station and a few minutes' drive from the M25, the auditorium is gradually filling to capacity. Predominantly female, predominantly upwards of middle-aged but with a visible smattering of far younger and far gigglier women, the Thursday-night crowd seems chatty and excited, and not particularly different from the audience you'd expect to see the following evening, when Cannon and Ball are due to hit Hertfordshire. There's little palpable sense of apprehension or emotional tension, certainly not enough to make you believe that those gathered here this evening are expecting to make contact with their dead loved ones.

"Lots of virgins in Potters Bar!" jokes Colin Fry, the spiritualist medium they've all come to see. He's just asked for a show of hands to establish who's been to what he refers to variously as a "demonstration" and an "experiment" like this before, and only a few have gone up. When I talk to audience members afterwards, though, it's clear that this was just a natural reticence that quickly melted; most are pretty literate in the ways of the psychic display and are familiar with Fry's work, either through his live appearances or television shows.

I am not. It's the first show of this kind I've ever been to; neither am I a watcher of the vast number of hugely popular television programmes (Fry's own Sixth Sense, Most Haunted, Psychic Detectives, Paranormal Witness). Inasmuch as I've ever thought seriously about this burgeoning industry, I'd put myself at the open-minded end of scepticism: broadly rational with a twist of "there are more things in heaven and earth"; pretty convinced that death is the end, with a fervent hope that it isn't (as long as the afterlife is nice); opposed to the exploitation of the grieving by fakers, but unable to see entirely how their made-up stories differ from much of organised religion.

But I've come to Potters Bar because Colin Fry and fellow medium Derek Acorah have asked me to; and they've asked me to because of what happened to Sally Morgan one late summer's evening earlier this year. Morgan is (in her own description) Britain's Best-Loved Psychic; her most celebrated client was Princess Diana, whose death in a car accident she almost foretold (she thought, at the time, that it was the Queen herself who was to die, although, as she points out, Diana did like to be known as the Queen of Hearts). This September, Morgan was playing to a customarily packed house in Dublin when an audience member sitting near an open window heard voices that, she claimed to an Irish radio station the following day, seemed to eerily prefigure the revelations that emerged on stage a few seconds later. The inference, and subsequent accusation, was that Morgan was being fed information about her audience, which she swiftly recycled as evidence of visitations from the spirit world, a charge she has robustly denied ever since – the theatre has supported her claim that the voices were technicians chatting. Although not, her critics have noted, to the point of accepting a Halloween challenge by science writer Simon Singh that she submit her powers to scientific testing.

"I don't want to get involved in the allegations," says Fry, when we meet up for a chat before he takes to the Wyllyotts stage. "It's not for me to defend that particular medium: she can do that for herself, or not." Interestingly, he doesn't mention Morgan by name, although he can't be talking about anybody else. Perhaps his natural demeanour – he is softly spoken, courteous, almost self-effacing – militates against such an out-and-out lack of gallantry; and perhaps it is also part of the positive thinking that he seems to apply to his work and its occasional travails. The Morgan brouhaha, he says, gives professional stage mediums an opportunity to open up their work for inspection, to reassure their public that "when I'm up there on the stage, it is just me, my audience, and whatever it is that I claim that I do".

Ah, there's the heart of the matter. What is it, exactly, that he claims to do? He responds very precisely, voice even more hushed, enunciation even more deliberate. "I have a perception of what many people think of as the dead. I have some ability, of varying degrees on different days, where I can sense the feelings and the thoughts and the words and the emotions and the memories of the discarnate. And it's happened all my life."

It began, he says, when he was a very small boy. When he was four, he told his grandfather that his mother had died and gone to heaven; the next day, his grandfather received a telegram to that effect. At 11, he passed a message on to his French teacher from her deceased mother; she told him he had a great gift but to be careful whom he told about it. Two years later, he'd realised he was a spiritualist and was also coming to terms with "something else the world was going to have a problem with – being gay"; by 17, he was demonstrating in spiritualist churches. But it wasn't until many years later, when he was nursing his adopted brother during a terminal illness, that he really began to think that mediumship should take over from his career in retail management. As Michael took his last breath, he tells me, he made the decision to go professional.

By the time I take my seat for the show I am intrigued, even a bit keyed-up. I'm eager for Fry to get through his humorous warm-up, the explanation that his hearing aids are not feeding him illicit information, his introduction of "a member of the press" ("Is that you?" the woman next to me whispers, spotting me scribbling in a notebook) and, most importantly, his entreaty that if we are not lucky enough to receive a message ourselves, we be happy for those who do, and take comfort in their comfort.

And with that he begins to listen out for the voices of the discarnate. Gliding around the stage, hand pressed lightly to head, he waits for the spirit world to guide him to their earthly relative (recently, he tells me, a dead woman called Dilys turned up 24 hours early, at the wrong theatre, to find her husband, and had to come back the next night). This evening, the spirits seem a little more accurate, particularly when they alight on a group of three Indian women, two older and one younger, who seem thrilled with information from their materfamilias; her grand-daughter is barely able to contain her excitement, even when she's being told to tidy her bedroom.

Throughout, the spirits seem tuned in to the prosaic, the domestic and the sad-but-not-tragic; one mentions lumpy gravy and tinkering with cars, another tells her daughter-in-law that her funeral was such a disaster that she forgives her for smirking at it. A distressed-looking woman is told that she has to take things easier and sinks back, relieved, into her seat. The process is relatively clear to me: it relies on Fry first narrowing down the audience, sometimes simply by gesturing to a part of the room, and then throwing out snippets of information that prompt individuals to identify themselves. The conversations that follow, as far as I'm concerned, neither prove nor disprove the existence of life after death, or the ability of the departed to communicate with those left behind; they're too general, too punctuated with hesitations and mis-steps, establishing details that fail to chime with anyone.

But something does happen that gives me an insight into what's going on. Just before the interval – before the punters disgorge to the foyer to browse Colin's merchandise, his books and CDs and Senses, his aromatherapy range – he casts his net once again. This time, a restless spirit is indicating that he should look for two people, a father and his daughter, or perhaps a daughter who's tried to persuade her father to come. The woman, he says, is called Jane, or Jean; paradoxically, his unsureness creates a sense of authenticity. But the words have an extraordinary effect on me. My mother, who died unexpectedly five years ago, was called Gina and suddenly, despite the fact that almost the last place on the face of the earth that I imagine she would choose to make contact with me is a theatre in Potters Bar, and despite the fact that I wouldn't dream of suggesting to my father that he put himself through more heartache in such an uncertain pursuit, I'm drawn in.

It's not pleasant: I realise that my face is wet with tears and that I have a sort of panicky, claustrophobic feeling, as if my mother needs me in some way but I'm powerless to help her; not entirely different, in fact, from the kind of feelings you have when someone you love is seriously ill or dying. Thoughts flash through my mind. What if I were to stand up? Would Fry imagine that this was some kind of elaborate journalistic sting? Would he help me? What if I sit here and say nothing and my mother thinks I don't want to speak to her?

In that frozen moment, I'm aware that I'm both elated and frightened by the thought that my mum might be here; and, impossibly, equally sure that she isn't. By the time I've begun to breathe more normally, someone else has claimed that spirit who, it transpires, was a woman looking for her son and her grand-daughter. I don't really hear what she has to say, because all I can think of is the glass of wine that I will knock back in one during the interval.

Colin Fry's brand of positive thinking extends well beyond his hope that stage mediums can survive challenges of the Sally Morgan variety. Much of his conversation tends towards the pastoral, the therapeutic: he says he often advises people to go to bereavement counselling, or to their doctor, if he feels that they're not ready to receive a message; he discourages people from visiting him too frequently if he feels it's preventing them from coming to terms with the death of their loved ones. What we must all realise, he says, is that we need to move on with our lives so that we are ready to meet those who have "passed over" when the time comes; and not to do so is disrespectful to them.

I ask him if he finds the constant company of the dead oppressive, and he says no, they are simply a "whispering, a half-noise" that is around him all the time. The real weight on his shoulders is the "duty of care" that he feels towards people, the responsibilities imposed by his gift. And yet he is adamant that the work he does is vital and that he has two questions for those who would legislate against mediums: "What would they give people in return? What are you going to replace me with?"

There's little sign that his devotees want him to do a disappearing act. When I speak to audience members after the final curtain (so to speak), they are delighted with what they've heard – particularly those who have received communications. Shehnaaz, her sister Sally and daughter Lucinda, who heard from Shehnaaz and Sally's mother, are bubbling with excitement, having made the long trip from Chingford by bus. And in the book-signing queue, Sally has received a further dose of good news about her health, which hasn't been great recently. Will they come again? "Definitely."

The only dissenting voices I hear are from a group of three women – another medium, a spiritualist healer and their very interested friend. And their complaint is not about whether Fry's communiqués are genuine, or even accurate, but whether there have been enough of them. At their church, they tell me, the messages come thick and fast; the jokey asides are kept to a minimum. As far as they are concerned, a spiritualist demonstration should have one purpose above all others: to provide evidence of the existence of life after death.

If the ladies from the spiritualist church thought Colin Fry veered too much towards entertainment, I'm not sure what they'd make of Derek Acorah, who greets his audience with a heartfelt "Love you!" and is rewarded with a rousing "Love you back!", and who spends very little time on the stage, preferring to range up and down the aisles in search of those ready to receive "a loving connection".

He is clearly a natural showman, as fans of Most Haunted, which he featured in for six series between 2001 and 2005 before a departure that was itself rather mysterious, will attest. But before small-screen fame, and before he turned to mediumship, he had another career altogether: as a professional footballer. At the age of 15, he tells me, he signed for Bill Shankly's Liverpool, causing consternation in his Evertonian family – and even more at the club when he started telling the other players their fortunes. After Emlyn Hughes had written off his brand-new car the day after Acorah had warned him to drive carefully, Shankly issued him with a stark warning: "You're an apprentice-professional with the greatest football side in the world, Liverpool Football Club. You're a footballer, not a psycho." It didn't, says Acorah, stop him asking whether they'd beat Villa on Saturday (they did, as he promised).

The vagaries of the sporting life – and dodgy knees – meant that he never quite made it, ending his playing days in Western Australia. Back in the UK, he planned on a new career as a coach, until the spirit world intervened in the shape of Sam, the "guide" who has been with him ever since.

There is something quite surreal about sitting opposite a grown man while he tells you about his adventures in a previous life; in Acorah's case, in Ethiopia, 2,037 ("2,038 in January") years ago. It was there, in a small village, that he first became acquainted with Sam, who was a local seer who used to tell the villagers "whether there was going to be a good harvest, watch out for marauding this, marauding that". Millennia later, Sam re-entered his life in a series of bewildering dreams, at the end of which he explained to Acorah: "You were allowed to follow your dream, but this is your gift that you should use to help mankind."

His response has been to help people via the "pure, unadulterated communication of connection", and by allowing Sam to guide him towards what he knows is "the truth, at a cellular level". Like Fry, Acorah is concerned that allegations of fraudulence "could set our sensitive work back 50 years, if people believed these accusations".

The people streaming into the Brook, an entertainment venue in the middle of a residential area in Soham, Cambridgeshire, don't look as though they've paid much notice to the nay-sayers (or, as Acorah calls them, "cynical minds"). As they take their seats in front of a large screen, on which images of Acorah at the Pyramids, or in front of his car, with its GHOST DA licence plate, mingle with notices for sufferers of epilepsy and advertisements for the True Vision tour and Soul Journey CD, they seem – like Fry's constituency – up for it.

But what "it" is proves, once again, elusive – as does his first taker. Eventually, after a certain amount of wandering, he finds a woman who recognises an Anne, who has a connection to George, or possibly Debbie, who is not tall and didn't worry about her hair. "She's a little bit annoyed," he tells the woman. "That's her, yeah." "She loved toast." "Oh yeah." And later: "Your mum's spending a lot of time around Mary." "She would do."

Unlike Fry, who maintains that he can't see into the future, Acorah is happy to let his audience know what's coming their way: in the case of a pregnant woman, that she's carrying a baby girl; to someone else that they'll find themselves in the dentist's chair the following week ("You've got a cavity"). And he delights in telling a man called David that, come Christmas, he'll find himself on a ship, although it's not particularly welcome news for David, who doesn't like cruises. In fact, of all the recipients of messages I've seen, David seems the most resistant, fighting shy of Acorah's tidbits and guesses – until the moment that he, too, recognises a name. When I speak to him in the interval, he confesses that he'd been sceptical until that moment, but could then not believe how close Acorah – and Sam – were to the mark.

Whatever impels Colin Fry, Derek Acorah and others of their ilk, it can hardly be the glamour. Before Colin's show, he and his crew regale me with stories of less-than-pristine B&Bs, tedious motorway drives and late-night sandwich suppers. When Fry goes to take a shower, he quickly rings down from his dressing room to ask if the management can bring up the shower-head, which is removed between each use, presumably to prevent its theft; when Acorah also enquires about the facilities, he's told he can have a wash and brush-up in a nearby caravan. By and large, their appearances steer clear of the big cities, the star venues; they focus on keeping costs down and ticket prices relatively low. And there are rewards to that approach. I ask one of the stage managers at the Wyllyotts whether mediums usually sell out and get an emphatic yes. Do other kinds of performers? An equally emphatic no.

In the end, I decide, there's virtually no evidence that either Fry or Acorah are using any kind of technical trickery to enhance their demonstrations; nor, indeed, particularly subtle forms of what has come to be called cold reading. This is a world away from the monstrous cynicism exposed by Hilary Mantel's novel Beyond Black or, more recently, AL Kennedy's The Blue Book, during the course of researching which she received advice from Derren Brown, himself a sophisticated analyst of the power of mediumship to convince those willing to believe in it. Instead, I think, it is a kind of updated end-of-the-pier show, performed by men and women with impressive theatrical skills and a rather touching connection with their audiences.

The million-dollar question is whether they themselves believe in their interactions with the dead; and that, unless you happen to catch someone in an act of fraudulence, is impossible to say. Is it harmful? Little more, I think, than all manner of other hocus-pocus, from horoscopes to new-age therapies. Would I go back? No. My fleeting wobble impressed one thing very firmly on me: whatever "loving connection" one feels with one's dead happens in the memory, and the heart, and the mind. I know exactly how my mother would have reacted to the sight of me in the audience in Potters Bar; with a mixture of laughter and sorrow and a compassionate instruction to take no notice. But she was not there. And wishing it were otherwise will not make it so.

Update: On 20 June 2013 Sally Morgan successfully settled her libel action against publishers of the Daily Mail, who withdrew the suggestion that she used a secret earpiece at her Dublin show in September 2011 to receive messages from off-stage, thereby cheating her audience, and accepted that the allegation is untrue. Her statement can be found here.