I remember the short-lived TV series in the early 70s, which I enjoyed that was based on this film, which was based on the book of course, but had never seen the film.
And so TPTV obliged and I have to say I quite enjoyed it, especially being a Tom Courtenay fan and it also starred, albeit too briefly, the gorgeous Julie Christie as a 1960s dolly bird up for anything (Bloody Hell - I've come over all peculiar and weak at the knees).
Oh, it also starred Rodney Bewes as Billy's mate and the whole thing made for an enjoyable comedy. Wilfred Pickles as Billy's constantly irate father was excellent too.