I'm happy to report that everything went well and the oven is working fine now. In summary if you want a main oven thermostat for a ZHQ575 check very carefully what your parts supplier is going to sell you. I successfully replaced mine with a 47THC9/F8
Wiring up the new thermostat was easy as it looks a lot like the old one and has the same number of connectors and similar layout and labelling. I wired the new connectors 1, 2, 3, 4 and 6 as they were previously. I left 5 unused, as before. The new thermostat has connectors A, B and C where the old one had F, G and H. I wired A and B as though they were the old F and G respectively, and left C unused, just as H was. I got the A=F and B=G part from the post below:http://www.diynot.com/forums/viewtopic.php?t=160349
The whole experience was long and frustrating and I nearly gave up thinking it might be wiser to buy a new oven. I'm glad I didn't as it would have been a waste of a perfectly good £500 oven which in the end I fixed for £40.
Thanks for everyone's input along that way. I hope this tale helps someone else.