Thanks guys for the suggestions.
It seems that the general concensus is that (a) it's too risky to use the
fuel, (b) there's no way to check it and (c) I should get rid of it and buy fresh.
The problem with just throwing it out is that
fuel is so blooming expensive these days (what with Iraq and all) that I'd rather use it somewhere. Someone mentioned I could use it in the lawnmower, but varnishing the cylinders doesn't sound too good.
Throwing it on the driveway might be an interesting idea (so long as no-one within coo-ee lit up at the time!!) but the oil mix would make a mess that might last weeks and be extremely difficult to clean. No??