Last Year Derek Jacobi, our most modest theatrical knight, decided he wanted to become a television star again, 18 years after 'I, Claudius'. It would "show people I was still alive." He wanted "something modern, in trousers," preferably something earthy in which he could use his native Walthamstow accent. As he turned down classical stage work, waiting for word to spread, he didn't know if a series would come along. It did, of course: a something in a dress, in sandals, with a silly haircut and requiring his voice at its most caring and cultured. But there was earth. He took it.

The part was Cadfael, monk, gardener, herbalist, detective, the hero of the 20 novels set in medieval Shrewsbury, by Ellis Peters, which have a cult following. Jacobi hopes (the first series) will be popular enough to lead to a second batch. There's a fair chance, he says, because detective series offer "a satisfying package" and this one's sufficiently different from the Morses and Poirots. And his role has charm. "Cadfael's not a goodie-goodie. He has lived out in the world, had adventures and responsibilities. He has done a lot of killing in the Crusades and known a lot of ladies. He had a son," he says.

Jacobi, knighted this year, was also impressed by Central's 12th century Shrewsbury, an entire village complete with a River Severn, re-created in studios just outside Budapest (at a fraction of the cost of doing a similar job in England). "The cathedral was so well made, we were all whispering in it." And the scripts include plenty of action, although when you see monks jumping off drawbridges and charging off on horses, don't look for Jacobi. "They wouldn't take the risk with me," he says ruefully. "All I got was pillion on a horse and a ride on a donkey with Sarah Badel leading me. We looked like The Nativity!."

If the series isn't ground-breaking, the actor whose 1977 Hamlet inspired Kenneth Branagh to go into the same business, is not complaining. "The bottom line is that it's television. It was fun to be in front of the camera, to be needed a lot of the time." The career restlessness may stem from middle-age melancholy. The East End boy whose mother worked in the drapery department of a store and brought home remnants for him to dress up and perform in, is now 55. More likely it's from "disappointment" (he'll put it no more strongly) that after 'I, Claudius', which remains a world-wide best-seller, the BBC ignored him. They paid him only a third of the fees John Hurt and Sian Philips received and offered him no work at all for the next five years. There must have been some corporate guilt, because a cheque for 600 pounds came out of the blue for "good conduct in rehearsals."

After Cambridge, with contemporaries including Ian McKellen, Margaret Drabble and Trevor Nunn, Jacobi knew his "heart lay in the theatre." But, he argues, "a bit of notoriety from films and television is a useful bonus; it puts bums on seats at Stratford or the West End." Despite dipping a toe in American movie-making water a few times, he didn't get it. He suspects he was difficult to cast. "I didn't appear to have the anger or the sex appeal. And I never really yearned for the fame" as did fellow theatre actor friends such as Anthony Hopkins. "Tony worked at it. He went out to America for ten years. That never appealed to me. I just waited for the phone to ring and for it to be Stephen at the other end," he adds, smiling.

The phone did ring when they were casting a role with maximum notoriety potential, that of beastly Hannibal Lecter in the film 'Silence of the Lambs', for which Hopkins won an Oscar. "I came close, but not close enough," Jacobi says.

This Spring he was back at Stratford, playing an earlier serial killer, Macbeth. He had no sooner told me that his hobby was "worrying about the next job" and that he would be out of work by July than that next job arrived. The news came through that he has been appointed artistic director at Chichester Festival Theatre. He insists he'll appear only in the opening production and then he'll be "available."