IF hindsight could influence our theatre-going, there was an Edinburgh Fringe show in the early-seventies that none of us would have missed. A bunch of Edinburgh University

Theatre Company unknowns was putting on a musical called Mandrake.

It was written by Mike Alfreds, who went on to found the influential Shared Experience Theatre Company, it was directed by Peter Farrago, who became a key name at the Royal Lyceum, and it starred Ian Charleson, the late Chariots of Fire actor, David Rintoul, of Doctor Finlay fame, and Tim Piggot-Smith, who most recently played the Alchemist at the National Theatre.

Also among the cast was one David Edgar, a man whose short-lived acting career was soon to be eclipsed by his playwriting.

Thirty years on from his Fringe beginnings, David Edgar reckons that in terms of earnings, he's in the top 5% of British playwrights. This might still put him some way behind your Alan Ayckbourns and your Alan Bennetts, but it's not bad for a writer whose success rests on a bedrock of agitprop, left-wing polemic and socialist ideology.

It clearly helped that he was the adaptor of The Life and Adventures of Nicholas Nickleby, the massive 1980 RSC hit that paved the way for the subsequent adaptation boom, but even there, his intention was to revitalise Dickens's social conscience in the light of the first term of the Thatcher Government, and certainly not to present a slice of chocolate-box nostalgia.

I meet David Edgar, a tall, soft-spoken and distinctly unproletarian 48-year-old, as he sits in on rehearsals for another adaptation - his treatment of Robert Louis Stevenson's Strange Case of Dr Jekyll and Mr Hyde at the Royal Lyceum.

It's his first visit to Edinburgh in 20 years. Last time he was here was in 1976 when the Traverse premiered his Saigon Rose, a satire about the American and English colonisation of Scotland, using a metaphor about venereal disease.

The then Glasgow Herald described it as ``bitter, bawdy, funny, lightweight, serious and very funny in turns'', even if in the end it was more of a ``near miss'' than a hit.

Prior to that production, the Birmingham-born Edgar had had a happy association with the city during a run of seven consecutive years on the Fringe. His first professionally published and produced play, Two Kinds of Angel, was staged in Edinburgh in 1970, and the following year two of his one-acters premiered at the now defunct Pool Theatre.

It makes a good deal of sense for him to be back now with Dr Jekyll and Mr Hyde, a novella set in a London with an atmosphere suspiciously similar to that of the Scottish capital.

``There's a theory that it is Edinburgh,'' says Edgar, who since 1989 has been lecturing in Playwriting Studies at Birmingham University. ``Certainly, the way the streets are described, it seems very empty for London. We know that Stevenson and a friend invented a couple of alter-egos who would roister round the Old Town, getting drunk. Hyde is drawn from that world, and I've made Jekyll Scottish.''

A prolific writer with more than 30 stage plays to his credit, and a similar number for radio and television, Edgar can't be accused of rushing this one off. His first attempt at it came in 1991 when it was staged by the Royal Shakespeare Company only to be deemed a critical flop.

Far from being defeated, the experience left him with a nagging feeling of unfinished business. His mistake, he decided, was to have split the characters of Jekyll and Hyde between two actors, drawing out an implicit father-son relationship, but making it harder for an audience to accept that they were different sides of the same man.

The solution was to go for one actor, and he persuaded Bill Alexander at Birmingham Rep that it was a play worth finishing. The new improved version was successfully staged in the autumn. Kenny Ireland at the Royal Lyceum had already signed up for a second production.

``Having a two-thirds satisfactory adaptation of a great novel is a commercial error,'' he says. ``It should be a play that's done a lot. After two or three years I still felt it was a good piece that had this flaw.''

Despite being a long-time campaigner for the profile of new writing, Edgar is only a fraction shame-faced for producing yet another adaptation. ``I do feel very jaundiced about adaptations,'' he says. ``In the eighties, the number of adaptations trebled as a proportion of the repertoire. And there's this absurd boom on television. It's indicative of a medium that lacks confidence in itself.

``It's slightly unfair for me to say it, having written Nicholas Nickleby which did lead to a lot of adaptations, but as a general policy it's rather dubious. There are some very good adaptations, but endless rehashes are an absurdity.

``In the case of Jekyll and Hyde it was an artistic need. It says lots of things about wickedness, evil and human behaviour that I wanted to explore. In that sense, it

wasn't opportunistic.''

Edgar is happy to describe his play as an entertaining thriller but, like Peter Arnott whose own adaptation was staged at Dundee Rep in October, the reason he gravitated towards Stevenson's novella was for the relevance of its moral debate.

As a confirmed political playwright, he doesn't feel he's had to make any special effort to make Stevenson's opinions apparent or to assert his own. What intrigues him is how, even compared to 1991, the story is ever more apposite to our times.

HE EXPLAINS: ``This is a version of the novel. There is a presence of the adaptation in the text - we start with the beginning of the novel done by two characters, so we put it in a frame.

``It is political in the widest sense, looking at a world of masculinity at the end of the nineteenth century.

``The play is about the failure of masculinity as repression; the failure of men to talk to each other; and on

a grander basis, standing

above morality and cutting yourself off from humanity.'' He continues: ``The contemporary comparisons are with homelessness, fear of the underclass, and the great syphilis scare.

``The two things that have happened since the play first went on is that we've had an upsurge in Hyde-type activity - evil, like the tragedy at Dunblane, has stalked back on to the stage - and there's been a renewed interest in split personality, through multiple-personality syndrome and suppressed memory syndrome, which have become prominent as reputable ideas.

``I think that's about an uncertainty about who we are. There are no longer any reliable models of ourselves as we embark on a new millennium.''

Two models of ourselves, in the changing shape of the actor Laurie Ventry, will be stalking the Royal Lyceum stage from Friday to February 1, as Stevenson's fin

de siecle meets Edgar's end of millennium.