It is still early in the day and Diana Rigg is wearing a pink tracksuit and no make-up. At 50, even in the unforgiving morning light, the glamour is still there. Ad Rigg is full of delight at having been on location in Wales for Unexplained Laughter.
"I loved filming down there," she says. "We stayed in this tiny market town and filmed up one of those secret valleys of which Wales has such a lot. Every time I crossed the border I was aware of being in a different country and it never ceased to excite me when I saw the Welsh signs coming up."
Rigg plays Lydia, the central character in Unexplained Laughter. She is a freelance Fleet Street journalist who takes refuge in her holiday cottage in Wales after splitting up with her boyfriend. Lydia is accompanied by her freind Betty (Elaine Page). They are chalk and cheese: Betty obsessed by food and always thinking well of other people; Lydia brittle, clever, self-centred and wicked, almost the embodiment of town valvues. In remote and rural Wales, Lydia encounters the Celtic spirit, symbolised by the wild, sardonic and inexplicable laughter of the play - uncannily, it has a palable yet mysterious presence.
"Lydia's a bit of a free spirit, very unconventional," says Rigg. "She's definitely over 40 and still behaving like a 60s swinger."
Unexplained Laughter is based on the novel by Welsh writer Alice Thomas Ellis. Is is characterised, by a pull between intense Catholic seriousness - a preoccupation with finding God - and barbed wit.
"It's a sophisticated ghost story," says Rigg. "The enigmatic laughter is partially brought to the valley by Lydia's discordant emotional state, rather as a poltergeist is activated in a house by an adolescent."
Despite the prickly screen relationship between the two women, Rigg and Paige got on well, surviving the rather spartan regime of filming. "Creature comforts were few and far between. We had one of those chemical lavatories but in the end nature had to do.
"We seemed to spend the whole time screaming with laughter because the characters are such complete opposites, and I suppose our own are too. Even our different heights - I'm nine inches taller than Elaine - make a good visual joke. Usually the only way to get us both into a frame was to find a hill and have Elaine standing further up the slope than me."
Rigg had been reading Ellis's novel (it was by her bed) when Alun Owen's script arrived. The part demanded fine-tuning between black humour and moral seriousness, but Rigg is adept at both classics and comedy.
"I love the contrast of going from Hedda Gabler to The Worst Witch, which I did for my daughter because it was her favourite book," she says. "I suppose the criticism could be levelled that I don't take myself seriously enough but I do believe that the key to this profession is enjoyment; if you enjoy doing something, the audience will."
Althrough Rigg throws herself into the part of the bitchy Lydia with gusto, at home in London she is quieter, more intense. There is evidence in every room of an astonishing breadth of readin. She sleeps with towering piles of books by the bed. Much-loved possessions crowd each other for space and family photographs jostle for attention.
This scholarly and bookish environment may lead one to think it is the home of a rather elegant, well connected don. Was it then something of a curse to have the laughter of Mrs Peel, the archetypal 60s swinger from The Avengers, still echoing in her ears, two decades later?
"I don't look on it as a curse," she replies. "Thank God one is remembered for something - and it wasn't absolute rubbish. But the 60s swung by me. I was getting up at 5.00am to go to film studios. I felt so out of it because the hippies deeply despised people who worked. That wasn't the thing to do. You sailed through life doing something artistic like painting a T-shirt."
Work is important to her, but not as important as making sure she has got her priorities right in terms of her career and her family.
"When my life was an open book in the 60s, because I knew no better, they got it wrong. And anyway, to talk about yourself endlessly is deeply boring."