IT is one of the great traditions of the theatre. Anyone who goes along to see Agatha Christie's classic whodunnit, The Mousetrap, is sworn to secrecy about revealing who the murderer is.

With the show about to open in Manchester, Louise Jameson isn't worried that the code of conduct is likely to be broken.

"We do ask the audience at the end of the show not to tell anyone," she said, "but when you think of it, what is the point in coming to a whodunnit if you know who done it?"

Louise is proud to be part of the play which is the longest-running theatre show in history, having played in London's West End for 60 years.

"Given it has been around for so long I find it astounding that people haven't seen it or still don't know the ending," said Louise, who plays Mrs Boyle in the production."

Having been with the tour for several months, Louise has come to understand its longevity.

"It's quintessentially British and in a way it's Agatha Christie at her best," she said. "It is quite formulaic. We're an isolated group of people and one of us is murdered and one of us is the murderer. We're also a group of eccentric characters. There's the newlyweds and the neurotic young man, the sergeant major and the grumpy old woman - that's me.

"But at its heart it is entertainment for the family and it is very engaging. Plus it has been going so long that it has become a period piece now."

Louise has long been one of the country's favourite actresses whether it be for roles ion the Eighties in series such as Tenko and Bergerac to playing Eleanor Glasson in the TV series Doc Martin or as Rosa di Marco in the top TV soap EastEnders. She has also had many theatre success both in the West End and with national touring shows.

Louise will spend eight months of the year touring major venues with The Mousetrap and admits that a long tour can prove challenging.

"I think we slightly hit a wall around the five month mark, but we've pushed through that now.

"Every actor will tell you that around five months into any tour you think you are going a little bit barmy. But because the audience response has been so incredibly warm and we've had that sold out sign outside quite a lot, we're now going great guns. It's a hugely successful tour."

A production like the Mousetrap is quite formal and leaves the actors no opportunities to deviate from the script.

"The skill with any tour is keeping it fresh but that is particularly true with something like Agatha Christie. Once you hit the right formula you then have to maintain it.

"But we've struck gold with this cast. That's not me putting a PR spin on things. We've got this kind of warts and all family love thing going on which is great."

The cast have devised a number of routines which have helped them become a tightly knit unit.

"We have something called the gin club which is a pop up party we hold after one of the shows every week," said Louise. "When you're on tour there's a chance you can go through a week of performances without seeing one of the understudies or one of the crew but because we do his pop up party, we touch base with everybody, which is incredibly important.

"Also one person takes charge of a venue, so we have a different head girl or head boy.

"It's their responsibility to organise a trip that is relevant to the area we are in. So far we've done everything from making pottery to visiting Roman villas to beach walks so that's been fantastic.

"I'll be head girl in Jersey, after all I did spend four years there doing Bergerac in the 1980s so I should be able to come up with something interesting."

To many men of a certain age, Louise will forever be associated with playing Leela, Doctor Who's bikini-clad assistant. Although she starred alongside Tom Baker for only around a year, it's a role which still sees her in demand almost 30 years later.

While she's in Manchester, she will be taking part in a special question and answer session with fans of the cult TV show next Saturday.

"The organisers found out I would be around and asked me along," she said. "It's going to be in the morning because I have a matinee in the afternoon.

"I really was in the show only briefly but I think I got many a young man through a difficult phase," she laughed. "I don't have the fur bikini but I do still have the second costume which was a bit like a leather tennis outfit.

"But Doctor Who has been good to me - I'm going to America four times in next 12 months all because of the show.

"It's a programme which attracts extraordinary mindsets it has to be said. Some of the most brilliant people in the land love it.

"Look at Mark Gatiss who has written for the show, that's how I met him and got to know him.

"But it's also for people who have been excluded or bullied. It champions the extraordinary and the different and has a very strong anti-bullying line through all its stories and I'm really proud to have been part of it."

With such a diverse career, it's interesting to hear Louise's answer as to what she enjoys the most.

"I know you're supposed to say 'what I'm doing now' but I really love doing pub theatre," she said. "It doesn't pay the bills but I find it really exciting."

As for a dream role Louise hints that maybe the best is yet to come.

"I'd love to do one big major movie, that's always just slightly eluded me," she said. "But new writing really excites me, so I think that the part I really want hasn't been written yet."

Agatha Christie's The Mousetrap, Manchester Opera House, Monday, June 27 to Saturday, July 3. Details from 0844 871 3018