Family life in the household of the legendary writer, comedian and poet Spike Milligan was an unpredictable affair. Prince Charles came to dinner (and recited poems by McGonagall until 4am); long-time business partner, collaborator and friend Peter Sellers did not. Dusty Springfield popped round; however, Harry Secombe (the third Goon) was absent. Children were sent to private schools, but exam results were not revered. Discipline was nonexistent, but daily routine was unassailable. Wives and children were acquired aplenty.

But deep, passionate, all-encompassing love was heartfelt and never in short supply. And if Jane Milligan (the fourth of Spike Milligan's six children) is anything to go by, you can't make assumptions about Spike's offspring either.

If you want a celebrity daughter trading on her father's name and living off his money, dripping labels and names, then this unassuming actor and musician is not the scion for you. Jane, 44, still lives in Barnet, in suburban north London, near to where the family grew up in a large but neat house, an "above-ground" pool their only nod to the trappings of stardom. Calm, unassuming, with salt-and-pepper hair and thoughtful blue eyes, Jane is at the heart of the Milligan clan (there are three other official siblings, plus two slightly unexpected ones) and tries to keep communications between them open. "Spike may not have been the most orthodox parent," she recalls, "but he was very present. He was not an absentee father. And he surrounded us with a fierce, unforgettable love. He may have died eight years ago, but I still feel that love now. I hear his voice. We all do."

That "full-on" fatherhood was very much thrust upon Spike in the early 60s – a time when men were still not expected to care for their children, particularly if they were tiny. When he divorced his first wife, June Marlow, in 1960, Spike was awarded full custody of their three children – Laura, Seán and Síle – then aged nine, six and four. Three years later, he married again, to Paddy Ridgeway, a musical theatre actor, whom he had met on the set of Invasion Quartet. "He chased her around a field telling her they would be married within a year." And Jane was born in 1966.

"We were such a tight-knit unit," she says. "Síle was so young when it all happened she didn't know that Paddy wasn't her mother, I didn't know that either. And Paddy was remarkable – she just took on this ready-made family, gave up her career and cared for Spike, who was at the height of his comedic powers, popularity and success." He was also having periods of manic depression, which fed his creativity and plunged him into terrible black moods.

"Paddy was a great homemaker and so good for Spike; they moved to Barnet and restored this beautiful house together. She even brought June back into their lives and found a 'place' for her, for my siblings' sake. And their social set included Princess Margaret as well as Eric Sykes and Neil Shand, the writers and performers and people of the day. I suppose they were a kind of groovy It Couple."

His fellow Goons – Peter Sellers, Harry Secombe and Michael Bentine – were not regular visitors. "They lived too far away, and anyway, their close friendship had really been in the 50s [The Goon Show was on from 1951 to 1960] when they hung out at the Grafton Arms [a pub in central London]. That was before my time, of course; after they had families, that changed and they became more like colleagues."

When Jane was six, she was playing with her mother and accidentally "banged" Paddy's breast. "It caused such terrible pain she was forced to go to hospital. I was in shock at what I had seemed to do."

In hospital, Paddy was diagnosed with breast cancer, at the age of 37, and had a mastectomy, the start of six years of treatment. In typical Milligan fashion, nothing was sacred. "We played football with her prosthetic breast around the bedroom," recalls Jane. In July 1977, Jane and Paddy left Spike behind with the elder three to go to New York for three weeks by themselves. "I know now it was her way of saying goodbye. We did everything together, from all the tourist things like the Empire State Building and Long Island to her letting me drive a car, and allowing me a grown-up drink. Thinking how she planned this big farewell makes me love her even more."

Paddy died the following February. "It was like a nuclear bomb exploded in our family," says Jane. "We kept things solid in practical terms [there was a long-term nanny who stayed with the family from 1968 until 1986], but Spike was broken-hearted. He had tried to tell me she was dying; nothing was hidden. And afterwards he cried and cried, and then expressed his pain through his creativity and writing."

Before Paddy's death, Spike had worked and slept at his Bayswater office in London during the week, returning to the family home at weekends. Now he gave that up, telling the children not to worry, that he would be with them all the time now. "So we had unlimited access to him," says Jane. "He gave us a wonderful childhood."

The days followed a strict routine. "Spike was incredibly disciplined. He would get up early, swim in the pool and have a cup of tea and a slice of toast, and then work all day in his bedroom. This was his bunker – the walls were lined with books on all his favourite topics – war, archaeology, jazz – and on the shelves on one side were every gift or drawing his children had ever made. He was devoted to us. He worked for us.

"In the afternoon, he'd take a short break, have another piece of toast and a cup of tea, and then we would all watch terrible TV in the evening and he would provide a hilarious running commentary."

Bedtime involved lots of storytelling. "He painted a wall in blackboard paint, and covered it in chalk-drawn stories and drew figures. He drew on that incredible imagination." One of her favourites was about a witch called Badjelly. When the story was published in 1974, the illustration of Badjelly that appeared on the frontispiece was "by Jane aged six".

Never "massively interested in academia", Spike sent his children to private schools but took them out during term time for wonderful holidays. "We went to Australia, to Africa, but it was fairly minimalist – all Dad wanted was a glass of good wine in the evening and somewhere to swim."

Behaviour was never a problem. Not that Spike was a disciplinarian. "After Mum died, I was so worried that I was never naughty. But there was one occasion when he had to put his foot down. I had let the bath overrun. He came hurtling up the stairs, looked at me and said with more exasperation than anger: 'Pull your socks up!' And that was it."

Jane knew instinctively she wanted to follow her father into theatre, but was wise enough to realise that drama school might be painful. "If you have a famous father, people make assumptions; they think you are grand. They think you are something you are not. I didn't have a lot of confidence in the first place, and I was still grieving for my mother. I couldn't face the reception I might get from other students if I did get a place."

Spike had not allowed his children to become arrogant: "He taught us humility; he had this incredible insight into how other people felt – and that rubbed off on me."

This despite his astonishing group of friends, which included Prince Charles. "He came to dinner around the time of his marriage to Princess Diana; just him and a bodyguard. We were all tired but he insisted we sat up until 4am reading aloud McGonagall poems." Spike had been responsible for "resurrecting" the poems of William McGonagall – noted for being the worst poet of all time. It must have been a hilarious evening.

Determined somehow to be in the world of theatre, Jane left school at 16 and found a job selling tickets at the Duke of York theatre, and then becoming a gofer for the impresario Cameron Mackintosh. She moved into sound production with The Phantom of the Opera and Miss Saigon before getting a surprise break – in traditional theatre style – when she was dragged from the sound booth by the writer-director Bob Carlton to replace the leading lady (who had fallen sick) in the West End production of Return to the Forbidden Planet. "Dad came to see me and he just looked and looked at me afterwards. I knew I was the image of Paddy, and I don't think he saw me. He just said: 'I haven't seen you for so long.' He wasn't looking at me, he was looking at her."

Spike had quickly married again after Paddy's death, to Shelagh, 20 years his junior, who he knew from the BBC. "We weren't surprised. He needed a wife, and something was missing from our home. But he asked me first if I minded, and of course I didn't. In honesty, I was still very numb at the time." But everyone got along well at the time, united by Spike's all-encompassing love.

Jane recalls her 15th birthday: "Dad asked me what I wanted to do, so I asked for a family dinner party in fancy dress." Being the Milligans, this didn't mean a few fancy hats; it was full-blown costumes. Jane dressed as a large punk bunny, Shelagh as Blunderwoman, Nanny came as an Egyptian, and Spike appeared as his old nemesis, Hitler.

In 1992, Spike called Jane to warn her that a Sunday tabloid was due to reveal that he had a love child. "He was devastated." At the time the child was conceived – a boy called James – after a one-night stand, Spike was coping with a desperately ill Paddy and mourning her already. "He sounded broken and sad, but I told him it didn't matter. I'd always had a strange fantasy about having a younger brother called James, so perhaps I had picked up snippets of adult conversation earlier."

Spike had cared for his son, buying his mother a house and putting James through school; he had just hoped the family wouldn't be embarrassed. But in the end, the story was sold, and a photographer tipped off.

Even so, the family embraced the idea of James as "another Milligan", though there was no fairytale happy ending. "I think he wanted to come and live with us, and thought we had some sort of Hollywood glamour life. I had to point out to him on the phone that, no, it was not like that, and that he still had a mum, which was more than I did. That our life was not perfect and sacred."

James, now 34, did come and stay with the family, and Jane is in touch regularly, although they are not especially close. "But he is one of the family, one of us – he is a Milligan," she says, firmly.

The children's ability to assimilate extra family members also came in handy when it emerged that there was a second unacknowledged child born after Paddy's death, Romany, who lives in Canada. Her mother was a journalist and artist who tragically died in Romany's first year of life. She was brought up by grandparents. Her reunion was less dramatic and not publicised. "She's been to stay too, and I like her very much. Honestly, who wouldn't want to have a big family and lots of siblings?"

When Spike retired from constant work (although never stopping entirely), he moved with Shelagh to Rye in East Sussex, where he found a cottage and finally installed the one luxury he had always dreamt of – an in-ground pool for his daily swims. "It was idyllic," says Jane, "and became the new family home." By this time he was heavily medicating himself for his depression, and his work, in Jane's words, became "stagnant". "He was his own apothecary," she says. "He'd take lithium, and I'd say, isn't that the stuff you put in car batteries?"

Heart surgery was needed in 1992, and Spike gradually began dialysis for kidney failure during the last two years of his life. "I was there a lot with him, sometimes just sitting in silence."

The night Spike died in February 2002, from liver failure, Seán, Síle, Jane and Shelagh were there – Laura was in mid-flight from Australia. "It was a terrible night – there was a full moon, and a thunderstorm, and a tree came down outside. It was like a warrior was dying."

Then came his funeral, a desperately painful day when the bell tolled, and Barbara Dickson, who didn't know Spike but had asked to sing at his funeral, prefaced her song by saying she had just been at the Scottish Parliament and "Scotland is in mourning". Dickson then sang a lament for a warrior. Spike's coffin was draped in an Irish flag. "It felt like a king had died."

Spike had not wanted to be cremated, insisting on a burial. The children, denied the epithet they wanted by the vicar, had it translated into Gaelic and then carved into their father's headstone anyway. "Dúirt mé leat go raibh mé breoite" ("I told you I was ill") now reminds mourners of Spike's anarchic wit and wisdom.

Now Jane has lost both parents, and lives modestly after inheriting very little from Spike's will, but she says: "I wouldn't have wanted more – events make you what you are. I have a big family, and I have five nieces and nephews." She has a partner but no children of her own.

She has extended her family through performing and working with children, particularly at the Chickenshed theatre group in north London. Currently in development is a play based around Spike's witch, invented for his grieving young daughter Jane 20 years ago, called Badjelly's Bad Christmas – a Spiked Christmas Pudding for this winter. "I brought Dad to Chickenshed and he loved it here," she says. "He loved the energy and the creativeness. So I was so happy to help get this production underway as a tribute to his incredible imagination and his sense of family."

Jane's life clearly still has a massive Spike-shaped hole in it. When she talks about him, her eyes suddenly turn from twinkle to tears. "There was so much love; we never went without that. He looked at life in such a unique and funny way – it makes it both easier and more difficult to cope without him."

Badjelly's Bad Christmas, 1 December – 15 January 2011 at Chickenshed theatre, Chase Side, Southgate, London N14, 020-8292 9222, bookings@chickenshed.org.uk or visit chickenshed.org.uk. Jane Milligan is appearing in Spend Spend Spend at the Watermill theatre from 9 to 25 September