Another update
I'm a member of a local WhatsApp group of folk who avidly work on and ride older motorcycles and they routinely mill, lathe, and otherwise make and repair bikes and engines. I asked for their advice and have ended up with similar recommendations from that group as from this group, with backup options if my repair fails.
One of the key recommendations is to buy helicoil style inserts from WTI Fasteners who even sell a complete kit similar to the one Steve Brown has
https://www.wti-fasteners.co.uk/product ... k-plug-kit They're a relatively old fashioned business, who take orders by phone. Prices are far better than many, roughly £45 + p+p for their kit including inserts for M14x12.5mm they have a double-stepped tap in their kit and an additional 10 helicoils is under £7 if I end up needing to repair lots of spark plug threads (let's hope not!).
So this is the route I'm going to take - helicoil as it using them removes less material from the head, and WTI fastener tools as the are well recommended and likely to be as reliable as the tool Steve uses.
More news once I've received the kit tomorrow. Hopefully the bike will be back on the road, rattle free, by the weekend
