update. so thanks for the advice. following your the advice above , i found out the TRV side has a very slow flow unlike the lock shield side. seems to be blocked in the TVR connection/shield. the TVR connection is the flow side and gets very hot but the water comes out very very slow/trickle into the radiator. Looking at the flow from the TRV side, am sure the connector there has to be replaced. the micro plastic pipe is also very hot all the way from wall to the TRV.
please see pictures.
Now the question is how do i replace that whole unit/connector on TRV side . Am not sure how to connect the plastic pipe to the TRV unit. is this something i can do with some advice please? am very confident disconnecting and reconnecting the unit to the radiator itself. just have no idea how to connect the plastic pipe to underneath the TRV. any info/ help will be much appreciated.
the Pin is not stuck. that has been checked and it moved freely up and down. however radiator is just warm.
It could be the pipework or the TRV. My system with the same symptoms (including the hot microbore pipe up to the TRV) was the pipework. The next step would be to remove the pipe from the TRV and see if water flows well through just the pipe.
Turn the system off. Shut off the TRV valve (this prevents any water from the radiator flowing out after the pipe is disconnected).
Put a cloth down (dirty heating water stains stuff easily), then place a receptacle (ideally a big flat tray) underneath to catch escaping water.
Undo the big silver nut and remove the stem elbow from the TRV. Some water will escape, but only under gravity pressure so it's easily stopped by a finger over the end of the pipe. Perhaps have an assistant handy in case you need to hold like that while they empty your tray.
If there's now a good flow from the stem elbow side then your problem is the TRV. If not then I'm afraid it looks like it's you pipework that's blocked.
(If by this time you're quite confident with how little water is escaping you can turn the heating on and see if the flow gets any better.)
My reply #14 of this thread explains how the Scottish Gas guy went about curing mine after proving it was blocked pipework, using the diagnosis method above.
The link to the garden sprayer in that thread doesn't work now and I can't edit it. Here's a similar one.