Thanks for all the great advice.
The short version is - it's off now, no problems once I got the right tool for the job.
And now for embarrassing idiot-confession time - I think the fundamental problem was I was using a 13mm socket when it's actually a 12mm head. D'oh. I didn't realise this until the thing was in my hands and I was checking the fit against my wrench. Have already started writing out 100 times, "I'm an idiot, I must not use the wrong size wrench or socket."...
I think my idiocy was a combination of: late in the day/tired so poor judgement, poor light, press-on-itis, poor access (the foot rest/fender bar thing is in just the wrong place, as is the front wheel...) and just being a bit thick. The good news is that I've acquired a few more shiny new tools!
I went with the "get a decent 6 point socket and a long handle (15 incher)" route first - unfortunately, the socket just went round and round (well, it would, wouldn't it - it was the wrong size!). So then I went for the desperation ploy - a Craftsman Bolt-Out "as seen on TV" kit. Worked straight away (once I'd got the right size on!). Once started, I tried the regular (i.e. one size too large) socket out of curiosity and it worked ok.
Have now removed the worst of the rounding off on the edges and I think that it should all be OK if I use the right size socket.
There is one more question now - what should I use to punch out the pin in the bolt that holds the oil filter bypass valve together?
It's a very small diameter pin, maybe 2-3 mm, and I've nothing small enough and strong enough...
Thanks again for the help.
