Could it be a fuel flow problem? , Blocked fuel tank vents, fuel pump problem, or perhaps air flow somehow lowering fuel level in the carb once you get under way.
I see you are in Guildford. Do you fly out of Fairoaks? I once had half of the Surrey police force out after I landed my Turb upside down in a field of barley after an engine failure, near the Hogsback! Long time ago!