So, just fixed the dropout.
Driled the old M6 thread out no problems with the supplied drill bit. Tapped the new M8 thread no problems.
Wound the helicoil in, all good. Broke the end tang off. THEN checked it had gone in cleanly. Oh dear, it hadn’t. The last half-loop of wire was cross-threaded, and I now had no way of turning the helicoil to take it out. Doh!
In the end, I managed to get an M6 tap into it which wedge in enough to push it through the hole, enough to grab the other end with pliers and wind it all the way through.
Took a couple more goes to get the helicoil in, and it had probably worn things down a bit as it was no-longer a tight fit. BUT, the 4th one has gone in and is staying in. Bolt tightened up to it, will find out if it creaks next time I ride it….