Why it's so poor
As a lover of the TV series the film version just falls flat in my opinion, for the following reasons:
1. The recycled material. The new scenes are quite good: I like the bits on the rugger field and when Rigsby's with the camp boutique assistant. But all the other stuff was like a rehash of cold leftovers.
2. The dire music. The old plinky-plonky tinny piano theme tune of the TV series fitted perfectly. The feeble attempt at a glitzy seventies theme tune ('Rising damp is gonna getcha!') is just embarrassing. Why not go all the way and have slide-whistle sound effects when Rigsby falls over.
3. The location. The original TV studio sets were great and added to the claustrophobic feel. The gloomy old Italianate house with its bizarre collection of Victorian furnishings (eg the skeleton in Alan and Phillip's room), creepy garage and overshadowing shrubberies were all part of the charm. But the film moves the location from a detached house in a Northern university town (Leeds?) to a brightly lit terraced house in London, which is just a standard 70s lodging house with no character.
4. The photography. Part of the charm of the TV series is that Rossiter's face was made for TV close ups. The film does everything in middle distance and we lose those great gurning facial expressions.