The heated variety fed by the combi boiler.
It has two adjustable valves, right and left.
To vary the temp, which valve is adjusted?
Should one be fully open and the second adjusted?

Thanks for any advice.

I’m not sure if you can vary the temp. They will most likely be flow rate controls. Fully open or wound out = max flow.

That sounds a little weird.  Photo of the valves?

Two little identical valves with no TRV?
Just open one fully, fully close the other then gradually open that one until desired temp

One side (normally with a smooth cover over the valve) is known as the lock-shield and should be used to balance the system. I.e. if the rad is close to the boiler it would be mostly closed, if at the furthest point it would be fully open. This ensures the water doesn’t circulate via the closest rad and leave everything else isolated/cold. You can balance by trial and error, but the proper way is to compare the input and output water temps.

The other side is the one you can adjust as you please, or can be replaced with a thermostatic valve. Often the towel rad is not fitted with a thermostatic valve so you can have warm towels even when the rest of the house doesn’t need heating.

If both sides of the rad are fitted with standard valves, I don’t think it matters which one is used as the lock-shield. Does the rail also have an electric element in it for warm towels in summer?

Thanks Spooky.

Your information has enabled me to resolve the issue and we now have a piping hot rad from bottom to top.

I love this place.