Auction finished at £331? Now, I can see someone stripping the bike down, getting it resprayed and making it into a very nice bike again for a couple of hundred quid, but it still seems too much to spend on a rusty barn-find. From what I've seen of
eBay auctions of Bromptons, it's pretty hopeless trying to get one for a reasonable price, they always go for top-dollar.
I have a folding bike although I haven't used it much; it's a Ridgeback Envoy; a rebranded Dahon produced under licence for less money. You can get Dahons for not too much money but the Ridgebacks are overlooked so they tend to sell at a low price.