Cold radiator - help diagnosing problem please

If you unscrew the head of the TRV, can you push the pin down, does it rise back?

Turn off all other radiators, turn pump to highest setting, what happens? Any gurgling? If so, see if it bleeds air now (set rads/pump back afterwards)

Take the head off the trv. If the rad warms up you need a new head. If the pin moves up and down (but not all the way out!!) and the rad is cold, give the valve a few smart taps with a hammer. If that fails, replace it all

The system is balanced (I believe) - I had problems a couple of years ago, but after advice here I managed to get it nicely balanced without any problems since.

I noticed that some suggest a stuck needle in the TRV that requires removeing the top of it to release. How do I do that?

Just unscrew the ring below the head, no need to drain down the system for that, the head will then lift off revealing the actuating pin.

If the pin doesn't move it may help to pull it out with pliers. Depending which bit is stuck.

" nicely balanced "

Does this mean the lockshield valves were adjusted ?. try opening the lockshield valve fully as well (pic 2).

Thanks a lot guys!

Managed to follow your instructions and unscrew the head, and the radiator immediately started heating up. The pin seems to go up and down when pushed/released, however, as soon as I screw the head back on the radiator goes cold again, so I assume the TRV has had it.

Again, many thanks.

Could it be possible that the head is turned one full turn too many. A stop would have had to be removed to allow this.

I'll have a look later.