Poor Tom"" is that old standby of domestic farce--the house-guest who simply will not leave, The Man Who Came to Dinner--and this overextended British black-comedy does extract a few moments of nightmarish exasperation from the one-joke situation. Tom, an over-age lab assistant and professional idiot who's been lost ever since his ancient mother died, moves in for a visit with old school-chum Martin. And, with Tom's increasingly irksome presence as a catalyst, the household is soon turned to a frazzle. Martin's wife Vivian, never happily married and itchy since her children have grown up, at first hates having worshipful Tom around, then sleeps with him, then becomes his doting surrogate-mother. (Tom requires lots of care and feeding--what with being accident-prone, appendicitis-prone, etc.) Young son Paul, who's been up to no good in Leeds with a snaky chum named Angelo, drops by, is annoyed to find his old room occupied, and makes the mistake of borrowing some cash from Tom. Daughter Eva, living with a second-rate painter whom she supports (to mum Vivian's dismay), can't understand why her folks put up with jerk Tom-who obviously needs some sort of counseling. And Martin himself becomes the most infuriated by Tom's presence. . . until Tom sort-of-attempts suicide (in reaction to Martin's fury), whereupon Martin becomes as slavishly maternal as Vivian. Implausible goings-on, then--with only the most obvious points about family relationships made in the process. But shambling, unflappable Tom is an infectiously doltish creation; and some of the confrontations have the laconic, addled charm of such British playwrights as Alan Ayckbourn (The Norman Conquests).