Posted the post on another forum too 'cos a good few of us lay up our Seareys for a couple of months over the Winter.
Seems this is now a common problem.
No, this was Winter Gas.
No, it was not bought just after a delivery, in fact it came from a virtually new gas station ( less than six months old) which has the latest envirosafe tanks and all the goodies.
No, it has no ethanol. Various formulations are supplied by the different manufacturers over here. I checked out the ethanol content of the various petrols as Rotax recommend low or no ethanol gas (something to do with seal life,) and Shell do not put it in.
(Just as an aside, in UK we had to close our station when receiveing petrol but here you can fill up while they are unloading into the tanks.)
Several replies on the other forum confirmed that this is a concern because modern petrols have additives apparently recognised as a common cause of various unhappy consequences. Also seems gas in plastic containers (which we did not have when I was sprogget) deteriorates faster that in metal.
Over here thay sell 'stabiliser' to be added when you expect gas to be stored for more than a month or so. Two months seems to be the accepted point at which problems start.
Of course you can be cavalier about this, that really is not the point nor the reason for post. I have been using cars and therefore gas for forty years and never before seen it as a problem. I could have has a nasty event for my ignorance. If you use mogas in your plane do not fly it on old stored gas, drain it into your car and let it fart its way home like mine. Put some nice new gas in your plane and fly safe.