I know that there were films of British sitcoms made in the 70s - it seems to have been a trend. I haven't seen many but I know a lot of them aren't supposed to be as good. I know the film of Are You Being Served? more or less recycled old plots from the series.
I haven't seen the Porridge film in full, but was it as good as the series? I can imagine with Dick Clement and Ian La Frenais writing, there wouldn't have been recycled scenes and jokes, not with their quality of comedy writing.
I have heard of The Likely Lads film - wasn't this quite different to Whatever Happened To The Likely Lads?? I know that the film came after both series, in around 1976.