Hi there Stan and Neo plus anyone else with any advice!
Here's an update, followed by the next bit I don't understand!
Bores honed out beautifully. New rings on the pistons, new big end bearings, and the bottom end all back together - feels right, pistons all at the right height and turning over nicely. So I'm happy.
As far as the glow plug goes, great advice. I bought a tap as you suggested rather than go straight for the helicoil. Got the tap from Snap On, ordered it online £4.40+vat and p&p - stunning value! And this is the good bit it has reclaimed the thread and the new glow plug tightened up feeling dead right.
The head was completely stripped so I lapped all the valves in and fitted new stem seals. I don't know much about hydraulic tappets (is it common to call them buckets?) but I cleaned them up and refitted them. I put the cam in following the Haynes manual and tighted up 2 and 4 bearing caps. Here's the odd bit - all the valves are open to some extent! I can only suspect that the hydraulic tappets have grown somehow? What's that all about? What do I do to sort it?
My sincere thanks again guys