We have a problem in that even when the hot water heating part is turned off the rads still get warm. The plumber has replaced the mid position valve with no effect and he is baffled.
get a better plumberhe is baffled.
Thanks for that, I'll pass this on.Ok, so the plumber was not baffled then, it seems. That's good he's made progress.
If the radiators are getting warm/hot only when HW is on (this is not what you said in your original post - or not what I understood from it anyway), then this is usually down to the HW not being the last tee on the return. Common problem and a quick google will show you why this can result in reverse circulation through radiators.