Who can you trust?

That question fascinated actress Niamh Cusack and attracted her to the leading role of ITV’s new drama, Loving You.

The programme examines the relationship between a divorced mother-of-two and her new live-in boyfriend, which seems ideal until it’s torn apart by accusations of sexual crimes against a child.

The subject is highly controversial and has never been so topical. Parents were recently threatened with a ban on photographing their children’s school plays in Edinburgh, and several celebrities have been linked to sexual crimes against children.

Cusack says: "Child abuse is every parent’s nightmare. It’s a tricky subject and this drama will provoke a lot of discussion, I hope. But this wasn’t ever meant to be a drama about child abuse - it’s about trust. The subject was troubling and disturbing, but it opens up a big question about trust.

"Who can we trust? Can we trust anyone? There are a lot of people out there trying to re-map their lives and asking who they can trust - especially when it comes to their children. Friends of mine who are on their own say one of the main problems is finding a man suitable as a stepfather."

She adds: "I don’t think many relationships can survive lots of finger pointing - that was what really drew me to the script."

Cusack, 43, is probably best known for her role as Dr Kate Rowan in Heartbeat - 17 million tuned in to watch her die of leukaemia in the arms of her screen husband Nick Berry in 1992 - and last year she filmed her final scenes as consultant Christine Fletcher in the hospital drama, A&E.

She is a member of the Cusack thespian dynasty - her late father was acting legend Cyril Cusack, her sisters Sorcha and Sinead are actresses, as is her half-sister Catherine.

Cusack’s long-term partner, Finbar Lynch, with whom she has an eight-year-old son Calam, is also an Irish actor.

In Loving You, Cusack co-stars with Scots actor Douglas Henshall, Keith Allen and Marianne-Jean Baptiste and she says each actor found the script thought-provoking.

And although her personal situation is very different from her character Chloe’s, both constantly worry about the welfare of their children.

She explains: "In the drama you see a neighbour take her child out of Chloe’s house because her boyfriend has been accused of a crime against children. And I’m afraid to say I would probably be with that neighbour. You can’t take any risks with your children - that’s the awful thing.

"I have yet to get over the hurdle of letting Calam out by himself but know that time is coming. Finbar and I want to know where he is all the time and as parents, that’s only natural and right. You just cannot be too careful."

Cusack now lives with Calam and Finbar in London - but she spent her own childhood in Dalkey, a dreamy village near Dublin, and was blissfully free from such fears.

Although her upbringing was unconventional (her father left his wife and six children when Cusack was nine and had a string of affairs, including one with Vivien Leigh) she was always well looked after and felt safe wherever she roamed.

She reminisces: "I ran away from home when I was four and went to the hairdresser’s, had a perm, then the local policeman walked me home. Sadly I don’t think that would be possible nowadays.

"Still," she jokes, "I don’t think I ever paid for that perm. It made me look like Dougal from The Magic Roundabout but it was the only free hair treatment I’ve ever had."

Cusack admits that playing her latest role left her emotionally drained, but is used to feeling exhausted by her characters’ lives.

She recently played a forensic psychologist in ITV’s two-part series State of Mind, and because the character was a former professional rower, Cusack took up the sport so she could play the part convincingly. And she hasn’t stopped since.

"I’m a member of an eight in a rowing club," she says. "But I am not going to tell you precisely where - no way. I do not want the gentlemen of the photographic press corps out at dawn taking pictures of me making a fool of myself in water."

As if sculling down the River Thames wasn’t gruelling enough, the actress recently embarked on a rigorous training regime to run the London Marathon to raise money for the St Joseph’s Hospice in Hackney.

"A friend of mine works at the hospice and when I visited recently I was blown away by the atmosphere there," she says. "They asked me to run the marathon, so I thought: ‘It’s now or never’.

"I’ve managed 14 miles so far and I’ll push it up to 18 miles soon. I know it’ll be torture on the day, so I think I’ll have a look around and then latch on to the oldest granny I can find and run alongside her.

"And if I can come in a few strides in front of her I am going to look first-rate."

Loving You will be screened on Scottish on Monday, 9.05pm