Radiator cold at bottom/one side how to unblock

One of our downstairs radiators is struggling to get hot.

At first I thought it was a balance issue, but I've opened the tap fully, and removed and freed the TRV pin.

On closer inspection the right hand TRV is getting really hot, but the left side valve remains cold.
The top of the rad is also warmer than the bottom.

So, from what I understand it is likely to be a buildup of sludge, blocking the path to the non-TRV side. Does this all sound correct sor far?

Before I get someone in to try pressure cleaning it, I was going to remove the rad and try clearing it out with a hosepipe. Is there any point in doing that without first putting a cleaner in, and am I likely to be able to shift whatever the blockage is with just a hose pipe.

Or if I do need to try 'chemically loosening' it first, how do I go about adding that to a combi system? I've only ever done it by adding to a tank.

Thanks in advance.

Is the lockshield valve ( opposite side to trv ) definitely open ?
If you turn all others rads off at their trv only ,does it make any difference to the rad in question ?

Its definitely unscrewed open. Obviously I cant tell if it is actually open without getting the radiator off, but I assume it must be.
Nope - I have turned off all other radiators that I can at the TRV (some of the TRVs are broken)

(The TRVs need replacing, but I intend to replace with smart ones, so I started closing the TRVs off just to prove that the TRV bodys work and that smart TRVs really would be able to turn them off)

Interesting that you say about turning off at the TRV end on other rads. If one of the other rads was turned off at the non-TRV end, would that potentially cause this issue? I've no reason to suspect any of them are, but if so it would be worth checking before I remove the rad and clean.

Thanks

You use the trv only ,as if you used the lock shields you would affect the system balancing.
Close both valves and drain your problem rad and remove it. Then open the lockshield to ensure water comes from it and its working OK. Do the same with the trv. Take rad outside and flush thru with hose pipe ,tapping it with a rubber mallet would help to dislodge any sediment. Make sure water flows freely out both ends of rad ( from both valve ports). Put all back together and test. You can add chemicals via any radiator ( empty water from it first). A towel rad is ideal as the bleed valve is at the top ,vertical . remove the bleed valve ,insert a funnel and pour it in.

Thanks.
Yesterday I had set the heating to frost protection.
So when I got in I cranked up downstairs only (they are on separate thermostats) with the other radiators closed.

That did get some heat through the rad, and the lockshield was also hot.

So does that sound more like a balance issue, or does it sound like I've just managed dislodge all of the crud?

Not had a chance to see if it performs normally now with all other rads running and without letting it cool right down first. I guess that will be the next test.

Thanks again.

Turn off all the hot radiators. Does the cold one get hot?

Balancing is not done by opening cold ones. It is done by restricting hot ones.