Wow ok, food for thought here! I had the head worked on and cam followers adjusted etc a couple of years ago, so I guess, potentially it could be something like you say Jeroen? Would it still start successfully though when used the same day?? Thats why I'm thinking fuel!
Very helpful Tony, thanks! I will have to see if I have any brake cleaner about somewhere!
Steve, it sounds very much to me like you could be the winner. Perhaps the jets are working loose or something then - I will have a good check tomorrow morning on that front. The float chambers were half full when I removed the float itself, I don't know but perhaps were completely full with the float in?! I will do more checks
I'm really enjoying this troubleshooting, thanks!