Towel Rail Hell

I have a 2 year old towel rail fitted by a plumber. It worked really well up until this autumn.
Although one of the pipes below the rail gets hot, the rail itself never does despite the fact that both valves are fully open. I've tried bleeding it (gets warm for half an hour), I've tried disconnecting and re-filling it (same thing). Oddly both pipes seem to be pumping water into the system rather than one pumping and one draining away.
Short of threatening it with violence or reading it poetry I'm out of ideas.
Any suggestions would be welcome.
(it seems to work in the summer as well as the I'm guessing its not connected directly to the central heating system)

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