“But he told me that Mamma Mia!’s company manager was on the phone asking if I was in the building because they had an emergency. I simply got told: ‘They need you there, can you run over to Mamma Mia! and go now’.”The reaction from the 1,100-strong audience when Parry, 35, stepped on stage was euphoric, with loud applause greeting her appearance. Mr Effeney said the chances of Parry being able to step in had been “one in a billion”, telling the Evening Standard: “Otherwise we would have had to abandon the show and refund everybody’s money.”He added: “It was a completely unique experience for the audience. When something like that happens and it works the audience love it.”View latest offers from Theatre Tickets Direct During the scramble to find a replacement for Deverill they had been told theatre management were doing their best to make sure the award winning show could continue, but few held out much hope until they managed to get hold of Parry.“There was huge applause when she came on stage,” said a spokesman for the Novello Theatre. “They were desperate to see the show so you can imagine the reaction.”Parry last played the part of Donna Sheridan on stage in 2013 – although she did reprise the role on a cruise ship last year – so says she was a bit rusty, particularly in the dance scenes.“Suddenly had to revert right back, so I was being pulled around stage and guided along by others as we went through the night,” said Parry.“It was a packed house and they were marvellous. I honestly wish I could have taped the reaction and had it on demand for whenever I’m feeling down. I think it’s a really good moment to celebrate the camaraderie of the West End, that 42nd Street wanted me to go and help out another show and that we could pull all that together for the audience.” When the lead in the West End hit musical Mamma Mia was forced off stage with a calf injury the Novello Theatre faced the prospect of having to send the audience home.Fortunately stage manager Philip Effeney remembered that an actress who had previously played the part at the theatre was now an understudy at another venue, a few minutes down the road.So it was that just 20 minutes after the show had “ground to a halt”, Steph Parry stepped into the shoes of the injured Caroline Deverill – making sure the show did indeed go on.In a further twist the show Parry had originally been preparing for that night – 42nd Street – is based entirely on the premise of an understudy’s rise from showgirl to star.Her appearance as Donna Sheridan in Mamma Mia! on Thursday night was doubly lucky, as Parry had only got back from a holiday that day in order to take up her role as understudy in 42nd Street, at the Theatre Royal Drury Lane.She said: “It was my first day back at 42nd Street after a holiday, and I was back to being a standby waiting in the dressing room tucking into some carrots and hummus. My company manager then gave me a call, so I thought I was going to have to go on at the Theatre Royal. The Novello Theatre, in London’s West EndCredit:Nigel Howard/Evening Standard Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings.