Worcester Bosch 230 boiler-rads on every time hot water run-heating is off?

We have just had a plumber fit a new diverter valve on our Worcester Bosch 230 boiler. (230 quid)
But the return pipe is still getting hot and the radiators are getting hot when we turn the hot wate rtap on, even thought the heating switch is off on the boiler (hot water switch on only). He thought it was the diverter valve and replaced that...but the rads are still getting boiling hot!!