Hi Dave,
I can remember the buses on my first visit in 1981. Seems like a lifetime ago but in those days the buses were all painted green and I think I might know why they were able to keep these grand old machines going for so many years.
During the late 60s when I was a student I used to work in a garage during spare time and hols to earn some money. I recall how 'Joe Bloggs' could work on his car and service, tune and sup up etc. I even spoke to one guy who said he used to replace big end bearings on one old 'ice cream' vending vehicle, by melting the metallic medium and 'lapping' it onto the crankshaft. I don't know if he was winding me up and I have never seen this done (maybe someone might know about this?), but if this is true then it's because these old buses can be repaired, and engines easily refurbished time after time that has kept them going all these many years.
Nowadays engine are so sofisticated that there is very little the you can do to a modern vehicle without involving computerised diagnostic tools!!!
Trev