Well for goodness sake! What was all that fuss about? You know my trouble - I think too much!
Drove to the local motor factors, who directed me to the engine rebuild place opposite, who sent me 200 yards down the road to a small industrial unit where the guy had EVERYTHING to do with mending stripped threads.
He lent me the tools, gave me a couple of inserts, and said pay me when you bring em back.
The job was done in half an hour, bike on centre stand, no drilling. Way simpler than I'd expected. Flushed through with half a litre of semi to get out any swarf, although most of it came out on the tap, which I'd greased. I thought the little tag on the insert was going to be tricky to remove, but a slight push and tug with long nose pliers and it was off. Works like it's meant to!
New oil, start engine, watch for drips, and ride into town to return the (cleaned) tools.
"Oh just call it a fiver" the guy said. I gave him a tenner and considered I'd got off lightly. What a gent.
I hope this reassures the next person here who strips the threads when changing the oil. It's a simple fix. Don't panic. Oh -- and, um, leave the JB Weld in the drawer........
Thanks guys for all the input. I'd never even heard of HeliCoil......
Here's the link. He ships all over the world, and has ALL sizes and threads, even arcane ones!
www.uni-thread.com