WITCHCRAFT (9pm BBC2) is the BBC's Yuletide spine-tingler. Nigel Williams, soon to become the new editor of Omnibus, has adapted his own novel into a two- part drama about the malign influence of a 17th-century witch- finder upon those engaged on researching his life in the present day. Eager for more background for a film he is making about the zealous Ezekiel Oliphant, Jamie (Peter McEnery) consults his old history professor Alan (Alan Howard). The academic has become so obsessed with his research into the witch-finder that he has even moved into his old manor-house. He talks animatedly to Oliphant's portrait and sleeps underneath it, curled up with his dagger. Murder, mayhem and a great deal of colourful acting ensue for those who meddle with matters beyond their ken . . .