JEAN Fergusson has no doubts at all that she has embarked on the the greatest theatrical challenge of her career. Fergusson, who has established a niche in the nation's hearts as the man-hungry Marina in Last of the Summer Wine, has spent the past five years researching the life of one of the great British comediennes and actresses.

And now she is bringing her one-woman show to Edinburgh. But Fergusson sadly admits: ``If you're under the age of 25 it's hardly likely that you'd even recognise her name . . . Hylda Baker.''

The comic died 10 years ago. She was alone, confused with Alzheimer's Disease, and estranged from her family. She had ``retired'' in 1976, when symptoms of her medical problems became all too alarmingly apparent.

In a Manchester cabaret nightclub, Baker tried to fulfil a two-night booking with her stooge, the famous Cynthia character (in reality a tall man, in drag, who never spoke) and totally forgot her all-too-familiar act. She was paid off by the management after one performance.

Emotional, exhausted, and dazed with the indignity, she slipped and fell on the stairs outside her dressing room. ``She grazed her face on the anaglypta wallpaper, and cut her knees,'' says Fergusson. ``She was lying there, crying, bleeding, her make-up all smudged. A tiny bundle of four-foot-ten has-been who people refused to book. She just couldn't be relied on . . . It must have been a terrible end to what had been a glittering career in the fifties, sixties, and early seventies.''

It was in 1991, that the idea of presenting Hylda Baker's extraordinary life came to her. ``I came off stage one night, and a colleague said, `You know, you do an awful lot of Hylda Baker mannerisms . . . and you sometimes sound like her, too.' I became aware that no-one had ever researched her life.''

Fergusson also admits there have been several spooky moments with the show along the way. ``I called in to see her niece,'' she recalls, ``and I told her that I needed some additional material, that indicated that Hylda had problems, that she was being extremely difficult. She'd brought out all the old scrapbooks of Hylda's cuttings, and she said, `There's nothing like that at all.'

``Something made me turn to the back of the book, where I expected to find blank pages. Instead, in Hylda's own handwriting, there was a full account of how she felt, how she was coping, what she feared, and her reaction to her illness - she clearly thought that she was going mad. No-one knew about the dreadful effects of Alzheimer's in those days. It was the most beautifully poignant piece of material I've ever read and gave me the key to her character in the later part of her life.''

Doncaster-based Jean was born in Wakefield, and then moved with her family to Bridgend in South Wales. ``Basically I was always a playground show-off. And one of my favourite impersonations was . . . Hylda Baker. It was in Wales where a friend of mine suggested that we joined a local amateur dramatic group, which had a pretty good reputation. ''

Turned down by both RADA and LAMDA Fergusson found a place at the Castle school in Cardiff (``who also turned out Anthony Hopkins, so they're not that bad'' she adds) and went on to carve a name in many of the leading rep companies of the UK. But it was as the outrageous Marina that she finally found national fame. ``A gift of a part,'' says Jean, ``and long may it continue.''

Like Hylda Baker before her, Jean slogged her one-woman show around Britain. ``Hylda was in the business since she was 10, in 1915, and she didn't become an `overnight success' until the advent of television for the masses in 1955.''

``She came from quite a large family, but she really cut herself off from them. She lived in London, in a 10-room flat off the Tottenham Court Road. She made three films as a straight actress, and she was brilliant. Her ambition was to play the nurse in Romeo and Juliet - she never did!''

``She was difficult,'' says Jean, ``there's no denying that. But that reputation came from her being a perfectionist.''

That may have - partly - led to the animosity between Hylda and Jimmy Jewel when they were teamed in the two top sitcom series of the sixties, Granada's Nearest and Dearest, and Not on your Nellie. ``Hylda used to `dry' and forget her lines, which totally exasperated him. She'd have cue cards littered out of camera-shot all over the set.

``She had no children - and that was part of her problem. She tried to adopt once, and they told her that she was too old. Married once, it failed after a short time - and then she had several relationships, none of which lasted. But she was, in the final analysis, a comic genius.''

n She Knows You Know starring Jean Fergusson will at the Pleasance for the Edinburgh Festival Fringe, August 7-31.