Gillian Anderson, Jim Broadbent and Damian Lewis appear in Park Theatre’s Whodunnit Unrehearsed

Running an unsubsidised theatre with multiple original productions a year in two auditoria requires an exhausting commitment to fund-raising.

So it's lucky that Park Theatre has generous ambassadors who rope in their celebrity friends for gala evenings and auctions.

Artistic diector Jez Bond has outdone himself with his latest charity scheme - co-writing a spoof murder mystery then getting unrehearsed star actors to appear each night as the sleuth who solves the case.

Gillian Anderson, Jim Broadbent and Damian Lewis are among the actors donning a trilby and an earpiece to be fed their lines during the thriller.

And the biggest mystery is that audiences won't know which celebrity is appearing each night until the start.

"It's one thing building Park Theatre and quite another thing running it," says Bond, who opened the Finsbury Park venue six years ago.

"We are a charity and we run at a huge loss every year. We need £300,000 just to keep the doors open. We can't fundraise that every year so every other year we have a gala night which takes a year to organise."

Instead of patrons, Park Theatre has ambassadors who are expected to be proactive, attend events and call in friends to help out.

Gandalf himself Sir Ian McKellen has given free performances to raise funds and supplies a voiceover for Whodunnit Unrehearsed, alongside Dame Judi Dench.

"Ian has a real commitment to grass roots and small regional theatres and has been an amazing support for us," says Bond who has co-written Whodunnit with regular collaborator Mark Cameron.

The setting is a classic 1950s murder mystery, an isolated manor, a body and a cast of characters that includes the Lord of the Manor, his wayward daughter, the housekeeper, and her son who is in love with the daughter.

Then a mysterious stranger arrives.

"We are totally spoofing it," says Bond. "There's the first murder, we set up the characters and then the inspector is called."

Clive Anderson, Gillian Anderson, John Bishop, Gyles Brandreth, Marcus Brigstocke, Jim Broadbent, Ronan Keating, Damian Lewis, Maureen Lipman, Joanna Lumley, Meera Syal, Catherine Tate, Tim Vine, Ruby Wax and Sandi Toskvig have all agreed to play the sleuth.

"The concept has never been done before, to have a person coming into a show where everyone else is rerhearsed and have them wear an ear piece," says Bond.

"They don't know what's going to happen, or what their cue is, they hear a line and then say it - it's really fun to watch."

Every night the celebrity is brought on at the start to be introduced to the audience and told how the concept works.

"The clever thing is that although the Inspector drives the plot and solves the crime - and feels like the lead - the number of lines means they can star without feeling as though they are playing Hamlet."

And if anything goes wrong, it can only add to the fun.

"It's bonkers, but they just have to go for it. If there are mishaps it will be funny, the audience will go along with it, we can't go wrong."

July 15-27 parktheatre.co.uk