Hot weather fuel problem?

Hi all. Got a problem which is puzzling me, in the heatwave we're having here in the UK at the moment (I know lots of you across the pond will be amused at the thought of consistent high-20s/low-30s C being a heatwave but we're not used to it in our little islands) my bike has started to leave a film of fuel on top of the airbox when it's hot. No evidence of leakage when the bike's just standing but when I return from a ride and leave the bike for a while, if I lift the tank the foam stuck to the underside is wet and so is the top of the plastic air filter box. I've no idea where it's coming from. I thought it might be the heat building up pressure in the tank but I've checked the breather - blown down the pipe OK, fitted a spare pipe and blown up into the tank OK and out with the cap open. Also checked the drain hose just in case, it drains from the filler cap surround and out onto the ground as it should. I wipe it down, leave the tank propped up and it dries out until the next time.

Any ideas?