Kara Tointon and Allen Leech on their new psychological thriller Too Good To Be True: A Cinderella tale... with a twist! A lowly cleaner, a rich businessman... but what's his secret agenda?

The four-part Channel 5 series is filled with twists from the first episode  READ MORE: Kara Tointon vows never to return to EastEnders

When Downton Abbey star Allen Leech returned to his native Ireland to film new thriller Too Good To Be True, you might have expected him to show his co-stars around town and enjoy a bit of after-hours craic. But when he realised the role required a shower scene, he embarked on a strict training regime instead.

‘Because I have to strip down a bit I was quite keen to make sure I stayed as fit as possible,’ laughs Allen, who lives in LA with his actress wife Jessica Blair Herman and their two daughters. ‘So I saw a lot more of the gym than the pub. It’s probably the most intense workout regime I’ve done.’

His role as Downton chauffeur Tom Branson didn’t require much bulking up, but as wealthy but mysterious businessman Elliot in Too Good To Be True he wanted a more chiselled look.

‘I felt it was important for the character,’ says Allen, 42. ‘Elliot’s very regimented and quite controlling, so I wanted him to look like someone who would stick to a schedule.’

The four-part Channel 5 series also stars ex-EastEnder Kara Tointon as Rachel, a single mum to young Liam.

Four-part Channel 5 series Too Good To Be True stars Kara Tointon as Rachel and Allen Leech as Elliot

To make ends meet Rachel works endless hours as an agency cleaner, so when she’s offered more money to clean for Elliot exclusively, she leaps at it. At first Elliot is charming – but then a twist at the end of the first episode makes you wonder whether he’s homed in on Rachel for a reason.

‘Until that point you really hope that this has the romcom vibe of a modern Cinderella tale,’ says Kara, 40. ‘But then you get a curveball and think, “Hang on a minute; this is getting dark.” As Rachel discovers more about Elliot, she becomes a bit of a detective in her own life.’

It’s Kara’s first lead TV role after a career as an ensemble player in shows like Mr Selfridge and The Halcyon. ‘In the past I tended to prefer theatre because I felt the roles I was getting were grittier,’ she says. ‘But with Too Good To Be True I could really get on board with Rachel and immerse myself in it.’

Allen says the plot will keep viewers guessing. ‘Rachel and Liam are such likeable characters that you’ll be really rooting for them,’ he says. ‘But what I like about the story is that it doesn’t hammer home the plot – from the first episode the audience is unsure who to believe.’

There were tough moments during filming for both Allen and Kara. Allen came off worst in a scene when Elliot gives a stray cat to Rachel and Liam. ‘The kitten scratched the entire side of Allen’s face,’ recalls Kara. ‘Luckily it happened on the last take before he was back in LA for a week, so he had a week to recuperate.’

Worse was to come for Kara during a fight scene two weeks before the shoot ended. ‘I tore my anterior cruciate ligament,’ she says. ‘It was after a morning of running around. All I did was put my leg on the ground and my knee just buckled.’

Too Good To Be True airs Wednesday 14th February 9pm, continuing Thursday 15th February 9pm. Pictured, Allen as Elliot

She spent the rest of filming with her leg in a cast, and camera angles had to be altered to hide it, with a body double used for some scenes. ‘It was a couple of months before I could really move the leg again,’ she says.

The good news for Allen was that once his scratches had healed and the shoot was over he could stop his training regime and enjoy a few drinks with the cast and crew. ‘Oh yeah,’ he says. ‘I blew the doors off at the wrap party!’

• Too Good To Be True airs Wednesday 14th February 9pm, continuing Thursday 15th February 9pm. Stream the full series on My5.