Whilst doing some bathroom work for us our plumber has added an additional radiator to our system downstairs. It only works when the hot water is on...

My uninformed brain says that he must have connected it to the wrong bit of pipe somehow but despite looking at central heating diagrams I can't work out how the hot water/radiator circuits work.

Can anyone give me any ideas?

Thanks ever so

Sounds like you are correct about the wrong pipe. You will need to track down the problem and make repairs or call the plumber back.