raden wrote:

When you said "remove the plastic nut" .. what did you mean ? With the
head off, there is no plastic nut visible. Instead there's a brass nut
which I can easily turn by hand.

I presume that'll be the one, is the pin located in a hole down the
middle ?

The plumber was here today and yes, the problem was found to be the
thermostatic valve which was stuck about 1/3rd open. Not only was the
pin stuck, the whole valve was seized fast - no amount of wiggling with
pliers or tapping with hammers/spanners was able to shift it. I'm
surprised about how this was able to happen.

As a "stop gap" the plumber has simply removed the valve (so now it just
runs direct) and balanced the return coming from the bottom of the coil
so that the radiators and the water heat "fairly" (as more or less would
have been the case with the thermo valve). I'm going to revisit the
matter in the summer and it's likely we'll stick on a motorized valve
and either a thermostat or a separate timeclock to allow independent
control. I'm likely to go the thermostat route as a few people here
advised same.

Thanks to all the helpful gents who patiently replied here - I've
learned a lot about CH/HW plumbing over the past couple of weeks. A
lot of newsgroups on Usenet seem to be overrun by trolls and morons -
it's pleasing to see some great people here keeping things moving nicely
with their expert advice.