Some background on the buses used in the films might be of interest. For each film, 4 ex Eastern National KSW5Gs were hired by Hammer Films from Syd Twell, a bus dealer at Ingham, Lincolnshire who, at the time, bought most of the buses disposed of by Eastern National, Lincolnshire and United Counties and some United Auto. I used to visit Syd's yard regularly at the time, and he said that in the first film one of the 4 was a bit duff, and in the 2nd, two of the four were. One of my friends in bus preservation owns one of them, and has videos of the films allowing analysis of the appearance of each bus by stopping the film. What this shows is that continuity is pretty non-existent. One classic scene shows (1) Reg climbing into a bus (2) Reg driving through the garage and (3) driving out onto the street. In each case, the reg. no. is visible, and it's different - i.e. 3 different buses used in one scene. All 4 seemed to move in the films but perhaps the duff ones were powered by the crew pushing it - in the series 'The Waltons' one could sometimes catch a glimpse of people pushing some of the cars as the Waltons 'drove off'!
The opening scene of the the last film shows the collision where they all got the 'sack'. The green KSW that is driven, with some force, into the side of one of the red KSWs was ex United Counties 939 (JBD977). This scene was at the film studios, but afterwards Syd's fitter just fitted another radiator and drove it back to Ingham. Incidentally, Syd wanted £250 for it before its film appearance. After the filming and when it was back in the yard I asked how much it was. Still £250 was the answer. Depreciation due to damage replaced by film star status
There was also a red LS DP in the crash scene. This also returned to Syd's yard, and it seemed to be a United Auto one though the tax disc was of a United Counties one. Obviously a completely accidental mistake.