This information from the Buses on TV Bus Station web site. To find out more info click on the link at the end.

On The Buses (London Weekend TV comedy series) Eastern National Bristol Lodekka FLF6LX/Eastern Coachworks were used. "The regular FLF6LXs were 2917 (AEV811F) and 2930 (AVW399F), with 2885 (WNO973F) and 2911 (WWC741F) also used - I do know that Brentwood depot drivers and Inspectors used to get paid as extras to be in the background, and I recall one inspector (George) saying he'd taken 2743 (EOO585) on more than one occasion."
A studio mock-up was also used: "YDU265D (fleet no 2552) was a mock-up of the front of a Bristol FLF built by Eastern National using parts supplied by Eastern Coach Works. Bob Palmer notes "the fleet number plates (2552) made up by Eastern National for the studio mock-up FLF still exist!" It was used for studio filming."(all the above from Chris Stewart).
David Ballantyne reports the use of ex-London Transport Leyland PD2 RTL1557 (OLD666), which was burnt - Steve Floyd notes "RTL 1557 (OLD666) was reported as being burnt. This happened in the season 6 episode 1 (20 February 1972 I think} called 'No Smoke without Fire'. The episode closed with Stan taking out '666' because there was no other bus available - after having been trained on fire extinguishers - but a fire started on the bus (whether accidentally or not I can't remember). Their efforts to put the fire out were unsuccessful and Jack dropped the fire extinguisher out the back of the bus as it travelled along a country lane. Stan drove into a field, as smoke billowed out the back of the bus. Blakey was rescued from the top deck by being assisted out the emergency window. There were several aerial shots showing the fire taking hold, and the resultant destruction - the last shot showed the upper deck completely gutted. I was surprised that the bus was completely destroyed - presumably it was bought for that purpose. But Stan's job was saved because Luxton & District got more than the bus was worth on the insurance!"
Incidentally, in 1973-4 there was an American spin-off show on NBC, called 'Lotsa Luck', starring Dom deLuise as Stanley, the custodian of the New York City bus company's lost and found department. More here.

Chris Brown notes "a black and white episode which has Stan and Jack having to work at Christmas. Olive drives the motor bike to collect them and crashes into a vehicle parked in the garage. This may have been filmed at Prittlewell, but whilst Stan and Jack are waiting for their lift, through the open garage door is seen one of the Bristol Lodekka FLF coaches. There is no sign of one of the regulars, 2917/2930.
I visited Wood Green during 1968/9 (it was when farebox MBS were operating at Wood Green so perhaps I am wrong on dates) and a Luxton and District Bristol FLF was in the Eastern National garage, and it was in use for sound recording, being driven around the area with the bottom of the stairs lifted and a microphone held by the gearbox. So 2917/2930 may have been the stars, but the noises heard were possibly from another one, as neither number appears in the list I made at the time. It is unlikely that fleetnames would have been applied if the vehicle was not to appear in any filming, but it definitely was not 2917 as I remember feeling disappointed at the time at not seeing it."