May just be a bit of muck stuck in the valve, would be worth having a look first, or even cut a lever valve into the heating flow.

S plan would require an auto bye-pass valve as well as system alterations

and why shouldnt you cut a valve into the heating pipe BTK  

Thats what they did in the old days. shut the heating down meant turning the valve off

Lot of system had valves on sections like the east or west wings

When I move dinto my current house, my rads became warm with the water on, mid valve also.

The plumber found that the bathroom rad (which was different to the rest, and plumbed in plastic rather than copper, so installed after the system) was plumber into the A/C.

They used the CH flow, but the HW return.  Took me ages to work out why it was wrong - drew a diagram, and all was revealed!

Thats what happened to mine.  There was three 22mm pipes to A/C  One is combined flow to mid valve, one is return from cylinder, the other is flow to rads.

The return pipe from the cyl ends up at the boiler, as does all the return from the rads.  They both just tee into the drop to the boiler.

As the 3 pipes where at the a/c, but no rad returns, the rad in the bathroom was bodged so the flow was on the CH flow, but the return from the rad was on the cyl return.

Sorry, Im no plumber, so hope that makes sence!!

I was offered the easiest option of having the rad on with the HW, or taking the return from the rad to the other rad return.