Jean Marsh glides into the rehearsal room and lets out a jaunty giggle. “When they told me The Times wanted me to do The Knowledge I thought they wanted me to learn to drive a taxi.” Sometimes meeting famous people is hard-going, like swimming through treacle, but I know immediately that Marsh is going to be fun.

The actress is a strikingly youthful 73 with a silky brown bob and an instantly endearing blend of dottiness and acuity. Only the lines on her angular face give the game away, but she still attracts plenty of male attention: “I was in a pub and these two young men came up to me. They said I looked like I’d had a hard day. I realised they’d just been watching a repeat of Upstairs, Downstairs. I said, ‘That was filmed 40 years ago, anyone would look tired. And by the way, shouldn’t you be at school?’ ”

To countless television addicts Marsh will always be Rose, the eternally dutiful maid in Upstairs, Downstairs. The series, which she co-created with her chum Eileen Atkins, ran from 1971 to 1975, and was a global hit, achieving the ultimate accolade – foreign fans thought it was made by the BBC, not the populist ITV. Despite the stardom, however, there is not an air or a grace in sight. She even travels to rehearsals by bus, using her senior citizen’s pass.

Marsh is back in period costume, this time on stage. She is about to appear in an adaptation of the Henry James novel The Portrait of a Lady, which tells the story of a young American heiress’s travels in Europe looking for Mr Right. Marsh plays her older companion, Mrs Touchet, to Catherine McCormack’s Isabel Archer. Portrait, along with A Doll’s House, opens the Peter Hall Company’s sixth season at the Theatre Royal, Bath and, as Marsh adds playfully, the script ought to be up to scratch as it is written by Nicki Frei, the current Lady Hall.

She is not quite sure how she landed the part. She thinks maybe Eileen Atkins put in a word for her when she worked with Sir Peter, or maybe he saw her as Mrs Ferrars in the recent television version of Sense and Sensibility. Whatever the reason, she is having a whale of a time, relishing the Katharine Hepburnish accent and loving her character’s independence, which echoes her own. The only catch is her corset. “I hate it rubbing up against my bones. But the designers insist on it to make sure you stand up straight. No way you can do 21st-century slouching.”

There is a nice irony in Marsh playing an American in Europe, because for much of her career she has been the European in America. Coming from a working-class family in Stoke Newington, London, Marsh progressed swiftly from rep in Huddersfield to playing Broadway with Gielgud, as Hero in Much Ado About Nothing. She is predictably modest about her success, putting it down to luck: “I’d just always worked. It just depends what doors you walk through.”

When Much Ado closed she spent a decade bouncing back and forth across time zones. “I did summer stock out there, just the same as I would have done rep here.” She had a small part in the blockbuster Cleopatra, played a black-clad existentialist in Tony Hancock’s The Rebel, did some Disney and The Twilight Zone before eventually settling back in England, where The Saint and Department S beckoned. She also worked with Alfred Hitchcock on his penultimate movie, Frenzy, and speaks very highly of him. “People say he treated women badly, but he was lovely to me. We talked a lot about food and wine.”

Everything changed with Upstairs, Downstairs. The idea was fiendishly simple, with Rose partly based on her former housemaid mother, who is 100 this August. It took a while to catch on, but soon there was no stopping it.

It made Marsh a household name in the UK and then, much to her surprise, in America, too. Again, however, she puts this down to good fortune. The series aired on the Public Broadcasting Service, which had only just changed its name from National Educational Television and was starting to attract decent viewing figures.

Marsh was the main beneficiary of the success, loved by both the public and the critics: “Our programme changed public broadcasting. When I won the Best Actress Emmy I said in my speech ‘Do you know how far it is from the East End to the West Coast?’ ” It made her name, even if she did not make her fortune. “£150 for the idea!” There is a big laugh and no sour grapes. “I get a bit more every time it is screened, but nobody back then had any idea that any programme would still be shown 30 years later.” She cut a better deal for her fashion house serial The House of Elliott, and enjoyed both the cash and the French and Saunders parody, House of Idiot: “There is nothing more flattering than people making fun of you.”

Marsh was chosen by PBS to be the face of Upstairs, Downstairs. “They put me on the poster. I was stuck on buses and rubbish bins everywhere.” It increased her pulling power, but made her uncomfortable. “When I appeared in Alan Bennett’s Habeas Corpuson Broadway with Rachel Roberts my billing was too big for my part.” This must surely be the first time an actor has complained about their name being too prominent, but Marsh had the same quibble again recently, when she was a very un-Rose-like maid in Boeing Boeing. “They wanted my name at the top, but I was quite happy to play third fiddle.”

Boeing Boeing marked a return to live work for Marsh after health problems. “I had a hip replacement and glaucoma, which was corrected. For the first time in 34 years I walked on stage and could see the audience, which was a nightmare. I didn’t realise they sat there waving their fans!”

Yet she definitely prefers stage to the screen, though maybe Sir Peter should skip the next sentence. “Once you get on stage you can do whatever you like, and unless they get a fishing net and pull you off they can’t do anything about it. There is a greater sense of freedom on stage. With films you have to do it again and again, and although it’s your performance the director makes you keep doing it until you do what they want.”

If she has any regrets it is not having any children, but it does not seem to be a major issue. She has certainly had no shortage of relationships. She was briefly married to Jon Pertwee and had relationships with actors Kenneth Haigh and Albert Finney and the director Michael Lindsay-Hogg. When she was with Lindsay-Hogg she found out she needed an operation to correct a gynaecological problem, but at 40 felt it was too late: “I think if I’d desperately wanted children I would have found out earlier.” After the only dark shadow during our chat I ask her if she is currently single. She sniggers like a teenager and says, cryptically, “Not quite.”

There is clearly an eccentric, playful streak to Marsh. Simon Williams, her co-star in Upstairs, Downstairs, says that she carries a herb rack everywhere. She promptly dismisses the notion: “Of course I don’t carry a rack . . . but I might take a chilli out of my bag if I don’t like the food in a restaurant.”

The Portrait of a Lady, Theatre Royal, Bath (www.theatreroyal.org.uk 01225 448844), Jul 3-Aug 9 2008

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