When searching Autojumbles it pays to be selective. The two big events which cater for pre-War machines are the one at the VMCC Banbury Run (next weekend, the 16th June) and Founder's Day at Stanford Hall, 21st July. It's always worth looking wherever you can but many of the regular jumbles are mostly post War stuff or mixed with modern Jap parts and car bits. Having said that the regular EJP Kempton Park racecourse jumble (next is 20th July) frequently turns up some good stuff. Also be aware that Enfield constantly changed the specification of all their bikes in the 30s, especially the tiddlers, so make sure you know what you're buying! Good luck!
REgards, Mark
PS anyone got a rear wheel complete for a 1934 Model S? It's for a friend, honest!