For storage you want to keep water and air out of the system. You don't want internal rust in the tank and lines. So filling it up is one thing. Next, gasoline does break down. Takes a while to do so. Varies on the purity and cycling temps for exactly how long. Bad gas stinks though. Smells like varnish and death. It leaves a thick gummy residue behind if you ever had a container sitting. But it needs water and oxygen to do so. Ethanol is more susceptible to drawing out water from the surrounding air so that's a no no. Don't know what's it like in the Uk but here they slowly starting introducing E5 and E10 to 95 octane. Supposedly those Fuel Tank stabilisers keep your gas from oxidising longer but they offer widely varying claims on how long and such.
Anyway, my song and dance for winterstorage as of now:
Battery on trickle charger
Clean oil and filter
Plugging up the exhaust
Tyres up in the air so they don't square off.
Oiling the cylinderwalls
And then something about the gas.