Rotax 912 Hot Start Issue

I have a Rotax 912 fitted to an Ikarus C42 which works fine.

The issue I have is if I stop the engine and wait for 15 minutes (to sigh customs book for example) it then won't restart.  After an hour when it has cooled it will then restart.  The Ikarus C42 is quite closely cowled and the carbs sit on top so they get quite hot, I've measured the float chambers at 50degC.  I've made a little fan unit so I can remove the coolant access hatch part of the cowling and use the fan to blow in some cool air.  This allows me to start the engine after 15 mins and go on my way.

I use MOGAS from Sainsburys (local supermarket in the UK) which is sold as E5 but may well have much more ethanol in than advertised.  I'm going to get hold of some UL91 and repeat my ground run test, which I'd imagine will then be ok but I'd prefer to use the cheaper MOGAS if I can.

I tried switching the fuel off so the fuel in the float chambers is used up before the engine is stopped, this has no affect.  Maybe becuase the new fuel was going through hot hoses and gets heated up anyway.

The vapor lock return is fitted and works, I hear fuel dripping back to the tank when the electric pump is on.  I've also fitted a fuel pressure gague to prove there is fuel pressure 15 mins after stop when the electric pump is running (electric pump isn't vapor locked).

I've also left the electric fuel pump on after stopping the engine to circulate cool fuel through the system to eliminate fuel vaporisation in the hoses as the cause.  Fuel was circulating back to the tank with the odd bubble in it.

The only cause I can think of is the fuel is getting cooked in the float chambers (fuel percolation).  Sounds like it should be a common issue but I don't hear of it being an issue for others.  I'd really like to find out how common this is, can I do something to help or do I need to put my hand in my pocket and go buy some proper fuel?