bathroom radiator

Dear Mike,
I have bought a house and the vendors did not tell us that the bathroom radiator did not work. It is the only radiator in the house that does not work.
I called out a professional plumber who told me that there was a blockage at the manifold and proceeded to rip up the landing floor only not to find the manifold and charge me £100.
I have taken the radiator off and flushed it through without problem. I also put cleaner into the system and drained it with no improvement.
There is good pressure and flow in one of the pipes to the radiator. There is some but lesser flow in the other pipe but only when the heating is initially switched on (presumably as water discharges from the hot water tank). After that the flow stops.
This does not strike me as a complete block as the plumber stated.
Can you please suggest something as taking a shower in the morning is getting pretty nippy.
Regards,
britprac.

It does sound like a blockage.
What about removing the rad.
Making up a fitting to attach to the rad valve.
With 15mm pipe inside.
Hose pipe and try with a hose from outside tap.
You may have to drain down first though.
If really dirty as in brown, then cleaning the system is needed.
Mike