Hi Dan and jujupunk.
Did someone mention the magic word - HIGHPATH?
There are no RetroBike rides in Bucks that I know of, apart from the annual Cleland reunion ride held on the first Sunday in December. This is organised by me and is intended for any riders interested in the early British school of mountain biking inspired by the bikes of Geoff Apps, who's Cleland Cycles company was originally based in Aylesbury in the late 1970s.
There are also other small rides organised from time to time but these are not promoted though newcomers are more than welcome.
By coincidence, the Rough Stuff fellowship (RSF) do have a section based at High Wycombe and they are holding a joint off-road event with the local branch of the Veteran Cycling Club next Sunday. I will be riding and there should be at least six Clelands being ridden or displayed.
If you would like details of this RSF event contact Francis on 07850 740591 for details.
The nearest RetroBike rides are those of the East Anglia and Lincolnshire group who have a ride on Saturday 21st July 2012 starting from Alconbury near St Neots Cambridgeshire.
I hope this info helps.