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.
