I'd be guessing it's gonna be a period of wasting time waiting to an engineer
My guess is either a valve that's fubar'd or an airlock somewhere ... However, Netto have those oil filled radiators for very little money if time or cash is short as an interim remedy.
(SpunkyMcSpunkbubble Diddled your nubbin on, Tue 23 Nov 2010, 20:49, 2 replies, latest was 15 years ago)

We have electric ones although the cost of running them is a bit excessive.
There is another valve on the other end that appears to be seized so maybe that's it? The pipe underneath is heating up, just not the panel. Could this be the lockshield valve Vipros mentioned?
( Davros' Granddad a voice of calm reason in a world of spastics., Tue 23 Nov 2010, 20:53, Reply)

could be
the lockshield one is a valve on the other end of the radiator to the one where you adjust the temperature.

generally it has a plastic cover that you pop off and adjust it with a screwdriver to balance the flow through and the flow past (or something)
( Vipros. clever got me this far, then tricky got me in, Tue 23 Nov 2010, 20:56, Reply)

If the pipe is warm then it's your valve that's jammed.
So twat it a few times, and then if that doesn't work replace it.
(Bazongaloid, Tue 23 Nov 2010, 20:57, Reply)

On it now.
Your mum, etc.
( Davros' Granddad a voice of calm reason in a world of spastics., Tue 23 Nov 2010, 20:58, Reply)

It's amazing, I never realised how similar radiators are to my mother.

(Bazongaloid, Tue 23 Nov 2010, 21:00, Reply)

Alternately warm and inviting then icy cold
and with a propensity to break down when you most need her?
( Davros' Granddad a voice of calm reason in a world of spastics., Tue 23 Nov 2010, 21:01, Reply)

I believe so
They're a son bitch for those plastic caps fraying ... Could always see if the raised valve bit turns with the aid of a pipe wrench or mole grips. By no means an expert btw.
(SpunkyMcSpunkbubble Diddled your nubbin on, Tue 23 Nov 2010, 21:01, Reply)

A new valve is about a fiver or so from Wickes.
Might be the easiest thing to change at first.
(Bazongaloid, Tue 23 Nov 2010, 20:56, Reply)