IT'S no classic, but for those of you looking for a break from 13 hours of James Mason in a toga in A.D. - Anno Domini, there is respite tonight in the form of David Carradine in Kung Fu: The Legend Continues (Pearl, 9.30pm). This is a welcome reincarnation of Kung Fu, the 1972 series also starring Carradine that kept ping-pong ball manufacturers and British comedian Benny Hill in business until it ended in June 1975. For the three years the series was broadcast, pubs throughout Britain reverberated to the sound of couch potatoes on parole looking cross-eyed into their beer and whispering ''aaah glasshopper'' at each other. The inspiration for this national mirth was flashback sequences that featured Keye Luke as Master Po, a Shaolin monk with ping-pong ball eyes who affectionately knew his young apprentice, Kwai Ching Caine (Carradine), as ''Grasshopper''. The plot was, to put it politely, uncomplicated. Caine, half Chinese, was studying under Master Po to become a Shaolin priest, but was forced to leave China after killing a man. As an adult he became an enigmatic and soft-spoken drifter who eschewed violence but, when given no other choice, was never afraid to use his prodigious martial arts talents to, well, break a few heads. Carradine, who has been making a living in Hollywood with supporting roles in unmemorable films such as Bird on a Wire, Armed Response and Boxcar Bertha, returns in tonight's show - an introductory film to the series that begins tomorrow - as John Caine,Grasshopper's great-grandson. The original Kung Fu, incidentally, is the series an unknown martial arts instructor called Bruce Lee claimed to have proposed to CBS executives in America in the 60s, with himself in mind as the lead. Lee claimed CBS passed him over for Carradine and went on to make a mint from the idea. In February 1986 a TV movie sequel was broadcast on CBS, Kung Fu: The Movie, introducing, in a twist of irony that now seems prophetically tragic, Brandon Lee as Caine's son. ATV'S alternatives this evening range from the educational to the ridiculous. Kingdom of the Crabs/ Devilfish (World, 8.30pm) is another immaculately-shot but rather unimaginative nature programme, this time an armchair tour among some of the strangercreatures of the deep, all without getting your feet wet. THE sickly Clara's Heart (World, 9.30pm) is a bad movie that Whoopi Goldberg starred in before she started starring in really bad movies (seen Sister Act and Made in America?). Ms Goldberg plays a cerebral domestic who works for an insufferable family of yuppies and their impressionable son, not in Tregunter Towers, but in Maryland, the US. It was directed in 1988 by Robert Mulligan, also responsible for a film with far better credentials, To Kill A Mockingbird, the 1962 adaptation of Harper Lee's novel. EVEN worse, but much more fun, is the enigmatically titled Hell Comes to Frogtown (World, 1.35am). Hardly surprisingly this was judged an outright turkey by critics when it was released in 1987. It stars wrestler Roddy Piper (universally unacclaimed for his roles in Body Slam and They Live) as convict Sam Hell. Mr Hell is the only fertile man left on our post-apocalyptic planet and is therefore chosen, lucky boy, to impregnate a bevy of women held captive by frog-faced gentlemen. The most notable feature of the amphibious protagonists, apart from their excellent frog masks, is that they have multiple genitalia, appendages that surely won't survive the censor's scalpel, even in a movie that was only 88 minutes to start with. It's enough to make you go cross-eyed and say ''aaah glasshopper . . .''