I also found a 240 page EASA document that debated everything about risks etc (due ethanol etc) and talking about how to mitigate. But it also contains copies of STCs that say I can use it. So with that and Chuck's info, I feel quite comfortable about legality of usage.
However, I'd prefer to stay with the aviation organisations that supply mogas (so it has the tighter specification), but airfields with pumps seem rare. Is there a current list of who has, and pricing?
Failing that, what fields do you know apart from Sywell? Thanks all! CT.