Thanks for all your help. I now know how to sort it out, but at a cost!

Every single TRV on the system was jammed shut and so old that when I
was messing around with one the rubber valve inside disentegrated and
started leaking all over the floor... luckily I had a drain on the
drain cock in case and I had started on the room with no carpets to be
on the safe side!

I believe what had happened was the boiler was just shutting down
because all the water was returning too hot, not having passed through
any radiators.

I'm sorry to have got you writing all this stuff when it was really a
very basic problem. I had allready removed the TRV heads and freed the
first pin, so I thought I'd done the neccessary, but it was really the
plunger that was causing the problem.

Anyway a lesson well learned. Anyone fancy taking the day off to help
me replace all the TRVs on Thursday? Hello? Hello? Anyone there?

Cheers