Thank your for your help.
If changing the bathroom radiator has upset the gravity circulation what can be done to put it back to what it was before?
Your explanation has been the most informative I have had as the plumber said he had never heard of such a system and as I said...
I have a Baxi Bermuda G F Super and it looks like 4 pipes coming out of it.
(The tank is downstairs in a cupboard in the same room as the boiler if that is of any useful information)
Turned off all rads except the bathroom turned on the water (without the pump) and the bathroom rad stayed cold. The pipe at the bottom of the rad coming up through the floor felt warm though.
Had central heating installed in '80's never had any trouble. When the water only was switched on the upstairs radiators warmed up particularly the bathroom which suited me. This year I had a new bathroom suite installed and as the old radiator was a single flat one decided to replace it...