towel rail.

Hi,
Just bought a house with Boilermate 2000 system which works fine apart from the towel rail in the en-suite. Upper level of towel rail is higher than bottom of header tank which is a worry. I've flushed the radiator and checked the valves and all works fine. There is water pressure at entrance pipe to radiator but nil at exit pipe. I called a professional plumber who said there was a block tt the manifold and to drain the system and clean it with Fernox. I have tried draining the system and drain only clean hot water presumably from the hot water storage.
If this is worthwhile how do I drain the system and then refill it to clean. Is this really a block or lack of head pressure? Presumably the answer to that is raise the header tank or replace the radiator with a lower one. If it is lack of head pressure would hot water circulate through the bottom part of the radiator? (the radiator is stone cold). If yes how much do I need to raise the header tank or lower the radiator? Thanks,
Britprac.

Raise the tank
Mike