i take it you found your rad thermometers then chaddy(at the ends of your arms?)

I read a link on here the other day about balancing the rads (I think it might have been on your thread chaddy?) but all other rads in the house seem to be at a similar temp (i.e. by touch), actually they are all bloody hot to the touch - maybe that is a problem in itself.

The gunk in the pipe theory is a sound one - the original high level heater was fed by 15mm pipe, and immediately after that it branched into 8mm - I may try taking the rad off tomorrow and give that a go.

The lockshield valve does have a drain cock on it, even though it's on the first floor (split level house - i.e. upside down), but it's it such an awkward position I may not be able to get a hose on it.

I've abandoned the "all other rads off" test now, as it's been 50 minutes with no effect. Shame - it was a good idea that made a lot of sense to me.

@ Chaddy: I don't know if the plumber who fitted the system balanced the rads or not. From what I can tell, there were only 4 radiators fitted when the house was built (1985), and sometime since then someone has been in and fitted more radiators (the pipes for these are 8mm and fixed to the wall inside conduit - looks terrible, but I have more urgent aesthetic problems to sort before I do that!).
So I think the whole system is a bit of a mish-mash of different people's work. I have been here for just over a year now so it's a mystery to me really.
The only thing my plumber did was to remove one high level fan heater from the kitchen. Unfortunately, I think he was just unlucky with this.

*shakes head*

I'd have shaken my head if i'd burnt my ear too

Woo hoo!
The plumber has just been and left. I have a new towel rail in my en-suite, my ball-valve in the loft is not dripping anymore and more importantly: the radiator in question is now working again.

Only cost me £40 so Dave.B is a very happy boy indeed!

The problem was what many of you suggested: air lock in the flow pipe going to the radiator. Cured as you suggested by closing one valve and cracking the union nut at one end, then vice versa. Just didn't have the guts to do it myself!

Many thanks to everyone who offerred their advice.
Dave

I know, like I say, I'm still beaming from ear to ear.
As I mentioned earlier, he has quit the trade and is now only doing small jobs for people he knows - he is a friend of a friend, so mate's rates apply I suppose.
I'm going to spend the money I saved on a slap up curry with extra sag aloo this weekend.