The actor Jason Watkins has a prominent tattoo on his left forearm; people he meets, he says, are often surprised about it. “When I’m getting ready for a part, the makeup artist will often say something like: ‘Oh, you don’t seem the type for a tattoo.’”

The tattoo spells out his daughter’s name: Maude. His wife, Clara Francis, has the same tattoo, only hers is on her right arm. “When we hold hands,” explains Jason, “we like to think Maude is right there with us.”

Maude died six years ago. She was two and a half, and all that seemed wrong with her at first was a bad cough. When it didn’t clear up, Jason and Clara took her to the GP, and later twice to A&E; by this stage, Maude was finding it difficult to breathe. Tragically, her illness was diagnosed as croup.

On New Year’s Eve 2010, the hospital sent the family home; Clara put Maude into her cot for the night. She appeared to be getting better, but when Jason went into her room a few hours later, she was cold – and he knew instantly she was dead. She had been killed by sepsis, in which the immune system reacts to an infection by going into overdrive, attacking the body’s own tissues and causing organ failure.

Jason and Clara’s daughter Maude, who died aged two and a half. Photograph: Jason Watkins

Losing Maude altered everything for the couple. “It changes you,” Jason says. “It changes everything in your life, and that includes your work. In one sense, after a loss this huge, work seems trivial. But in another sense, it becomes more important. I feel that whatever I do, I’m going to really make it count …”

And he has. Now 50, he won a Bafta for the 2014 ITV drama The Lost Honour of Christopher Jefferies; he dedicated the award to Maude. Then came W1A, the BBC self-spoof, in which he plays director of strategic governance Simon Harwood; and two movies in the successful Nativity series. This year, he landed the part of forensics expert Tim Ifield in the acclaimed TV series Line of Duty, and a part alongside Diane Keaton in the movie Hampstead. People who don’t know his name say “Oh, him!” straightaway when I show them his picture.

But he was less established in 2011 – and Clara, an actor as well as a jewellery designer, was, like him, freelance. So their daughter’s death plunged them into financial difficulties. He had been about to start in a West End play, and initially tried to continue but found it was utterly impossible to do so. And this, he says, is one of the many unacknowledged difficulties of loss, especially loss of a child.

“Suddenly there was no money – so on top of everything else, we had that to worry about.” Looking back, he can hardly believe he went back to work so quickly – he began filming on the sitcom Trollied four months after Maude’s death, and it certainly wasn’t easy. “There was a day on set when they had a baby doll. When they had finished filming the scene, the doll was just left lying there – I had to ask someone to take it away.”

We are chatting in the couple’s north London flat, and Clara is next to Jason on the sofa. While he hurried back to work after Maude’s death, Clara says she was incapable of anything but existing for a long time afterwards. “I was jealous of Jason,” she says, “because he could go out and he could be someone else. I didn’t have that luxury. For a long time, I couldn’t not think about Maude – she was all I thought about for the whole of the first year. I don’t think there was a second in that time when I wasn’t trying to process what had happened.”

We asked the counsellor how we’d cope. ‘What is death?’ he said. We wanted to kill him

Eventually, she went back to work. “The first day I just cried and cried: there was a puddle of tears on the desk. I was thinking, I’m never going to be able to work again. It simply won’t be possible.”

By this stage, she was pregnant again. “The day Maude died, I knew I had to have another child. It was primal. I didn’t want our older daughter, Bessie [who was four at the time], to be alone. But of course what I really wanted was to have Maude back, so when we found out we were having a boy, I was devastated. I’m ashamed of that now, but it’s true.” Gilbert, their son, is now five. “Having a boy turned out to be brilliant, the best way things could have worked out. But he’s here because Maude isn’t, and that’s a weird legacy for him.”

However desperate you are feeling, there are always ways in which you can get help

One of the things that helped Clara and Jason cope after Maude’s death was being part of a support group for bereaved parents called Slow (Surviving the Loss of Your World). “There were things we could say in the group that we couldn’t say to one another,” says Jason. “Every time we went to the group, we came away closer as a couple.”

Now they are hoping to pass on some of what they have learned through Jason’s new role as patron of Child Bereavement UK. “Losing a child is totally overwhelming: you have no idea how you are going to carry on, how you will look after your other children, and how you will keep your marriage together,” he says (he has two adult sons from his first marriage as well as his children with Clara). “But, however desperate you are feeling, there are always ways in which you can get help, and ways you can find to get through it.”

It doesn’t always feel like that, though: Clara relates how, in the immediate aftermath of Maude’s death, the couple saw a counsellor. “We asked him how we were going to cope, and he said: ‘What is death?’ And we wanted to kill him – because what we wanted was for someone to say it’s all going to be all right. And no one could do that for us. Now we know he was right – it was so hard to hear at the time, but now we know Maude lives on in our memory.”

They have campaigned for sepsis to be more widely understood. An inquest into Maude’s death concluded that no clinical mistakes were made. “It struck me that a system of care that didn’t acknowledge the possibility of a fatal condition was not fit for purpose,” says Jason – and he became involved with the UK Sepsis Trust. It recommends that any child presenting at a GP’s surgery or at A&E should be tested for the condition if they have or have had an infection of any kind. The trust’s campaign brought about a resolution in May from the UN and a global sepsis awareness campaign.

Clara says she, too, now feels more confident than before, and far less concerned about what she would consider the small stuff of parenting. “I don’t care about how well Bessie does; I’m not going to drive her nuts saying, ‘Do your homework.’ All I care about is, is she happy?”

She and Jason worry about the effect on Bessie of Maude’s death; the two little girls shared a room. In the months afterwards, Bessie would see Clara crying and say to her: “You’ve still got me. Aren’t I enough?” And that, says Clara, would trigger guilt. “Losing a child affects everything, every relationship you have,” she says.

Time does make a difference, however. “When your child dies, it’s like this massive red dot in front of you,” says Jason. “And as time goes on, it’s still right there in front of you, but it gets smaller. But the thing you know is, it’s always going to be there; it will never disappear, and that’s the right way and the only way for things to be.”

• Child Bereavement UK supports families and educates professionals when a child dies or is dying, or when a child is facing bereavement, childbereavementuk.org. Slow is at slowgroup.co.uk