One of my radiators is not working. The pipe up to the individual radiator thermostat is hot but the radiator only gets luke warm. I have had a plummer around he put a new thermostat on the radiator, charged me a frtune but its still not working. I can't afford to get him back. Any ideas?

Is this a steam boiler or hot water ?

If the plumber put on a part to fix your situation the return service call should be for free since the work he did is not working.

It is a combi boiler

You could have air in the system.

Have tried turning all heaters off except the affected heater, it did heat up, but its now just lukewarm again

You could have air in the system

HVAC has said it twice now, have you tried to bleed that radiator?