Emptied fuel tank put in 2 cups of meths (to absorb the water) and some fuel.
Engine ran for a few seconds.
Then ran for a bit longer and so on until it ran for hours. Been using it ok today apparently.
Bad bit is that I've been told he is probably not going to mention it to the hire company which I agree with a previous poster, is morally wrong.
If it was my engine, I always believed a rebuild was necessary (a) because that is what I've heard from others before this thread stated otherwise and (b) it's not difficult and I get to actually see that all the bearings etc are clean and ok. (plus I like taking things apart and rebuilding - but I am re-thinking that one now).
Anyway, I thought the meths idea was clever - not mine I'm afraid - thought I'd pass it on.