Thanks John, I might try that. I've tracked down all the Britton articles and compiled them into a word document. He does make heavy weather of some things, and much of it is frame stuff which has no interest for me but useful all the same.
Something else I've found really useful is a set of youtube videos by a bloke called Michael Waller. He built a Triumph Otter so rebuilt the (500cc unit) engine which again is very useful to me. He's here:
https://youtu.be/Q4tNS8rHKmYI've weeded out the Triumph parts. I've found almost enough to build a 350 engine and have sold a fair bit of surplus on ebay and elsewhere. I'm taking the residue to HJ Pugh who will include the bits in their April auction.
In other news I've been visited by a Greeves Riders' Association machine examiner who's happy my bike (with its Villiers engine) is pukka and the club is in the process of issuing a dating certificate so I can register it. I need to keep it more or less standard until I've gone through that process.