Brigit Forsyth is telling me how she missed out on a holiday in a Sardinian villa with her grandchildren to take up her latest role.

But she doesn’t seem too upset.

“There was really no option,” she says cheerfully. “I mean, it’s Bennett, at the National, with my good friend Sian. I told myself ‘Don’t be silly, you have to do this!’.”

The role is in Alan Bennett’s play People, a National Theatre production going on a UK tour. She stars alongside Sian Phillips, who plays aristocratic Dorothy, the owner of a crumbling Yorkshire mansion. Brigit plays her loyal companion Iris.

The tour kicks off at the Birmingham Rep on September 3 for three weeks.

“It’s just wonderful,” enthuses Brigit, who shot to fame as Thelma in Whatever Happened to the Likely Lads?

“I have never worked for the National before and I have always wanted to, and I love Alan’s plays.

“I have a fantastic part in this, though it’s rather different from the last time I was in one of his plays. Then I was the Queen in Single Spies, about the double agent Anthony Blunt who was Surveyor of the Queen’s Pictures.

“The Queen pays an unexpected visit to the gallery and is looking round at her objects. One of my favourite lines of all time is when she says ‘This ostrich was given us by the people of Samoa. It hasn’t quite found its place yet’.

“Alan writes so many amazing lines, and he’s like that when you meet him. I could listen to him talk all night, he is heavenly.

“Quite long periods go by in People when I don’t say anything, but I will suddenly chip in with one line that’s a killer.

“That’s actually quite frightening as I will have to make sure I stay awake. If it’s a hot day, it could be quite dodgy. I must not fall asleep, ever! My outfit is to die for, just layers and layers of Oxfam clothing because the house is freezing – shirts and skirts and striped socks. I can hardly wait!

Brigit Forsyth and Sian Phillips in Alan Bennett’s new play People
Brigit Forsyth and Sian Phillips in Alan Bennett’s new play People

“I have an amazing wig, which looks like something a dog has chewed up. It’s yellowy grey and horrific.”

Iris looks like a hunched over pensioner on stage, but Brigit – without the wig – is full of fun and doesn’t look or sound like she is 73.

“I know, I can’t believe it either, it’s a bit of a shock! I think I stay young because I’m in a very childish profession, pretending to be other people and getting paid for it. Acting is such a great job for stopping you noticing that you are crumbling. We’re so busy having fun.

“The bad bit is when you are not doing it. Real life is a bit boring if you get too much of it, I don’t know what to do with myself.”

Brigit has recently found a new string to her bow to occupy herself – playing in a band.

It was while starring in Calendar Girls in 2008 – on tour and in the original West End show, alongside Sian Phillips which is how they became firm friends – that Brigit formed a band with some of her fellow cast members. She plays the cello and sings in The Fircones.

“We do gigs all over the place in London,” she says. “I’ve played the cello for years but I never thought I’d be a rock chick in a band. Better late than never.

Brigit had two children with her late husband, TV director Brian Mills. Ben “has a proper job” as an engineer while Zoe Mills is an actress, also appearing at the National Theatre in Punchdrunk’s production of The Drowned Man: A Hollywood Fable. They’ve given her three “gorgeous” grandchildren, aged eight, six and three.

One of the themes of People is the National Trust, as Dorothy’s sister is determined to give the mansion away to the charity but she is firmly against people traipsing through her home.

“I used to be a member of the National Trust,” remembers Brigit.

“I think it’s admirable what they do. Thank goodness they restore places.”

Brigit last appeared at Birmingham Rep in Noises Off in May 2010, one of the final productions before the theatre closed for refurbishment. And now she is starring in the first play to reopen the theatre in September.

She returned to Birmingham more recently to film Doctors – “my first nun!” – and rediscovered an old haunt.

“I ate the best Indian food I’ve ever had at Shimla Pinks on Broad Street.

“I also remember being given a special tour of the Barber Institute art gallery, which was terrific.”