Why does my boiler not turn off?

Hi. I have a Potterton Ultra DV40 boiler and also a Megaflo unvented water heater. When the boiler is heating the Megaflo, the burner goes and and off as the Megaflo zone valve opens and closes. However, the water in the boiler continues pumping internally in the boiler. I understand this is due to an override valve that keeps the water pubping around for a couple of minutes to reduce heat. However, the issue is that this never goes off. I have to turn all the electrics off at the master switch to stop the pump. If I switch the electrics back on, the water starts pumping around the boiler again (but at least the burner stays off and only comes on if I open the Megaflo valve).

Conversely, if I turn the boiler on to heat the CH system, the burner itself actually stays on after the the CH control tells the system to switch off. The only way to stop the burner in this case is to turn the thermostat on the boiler down to zero. If I switch all the electrics off to stop the boiler completely and then switch back on again, the burner starts up again when it shouldn't.

So in summary, turning the boiler on at the master switch always results in the pump starting up. If the boiler was previosly heating the CH loop, the burner also comes on.

Any ideas anyone ? If anyone points me to the problem they'll get be a big thankyou as my gas engineering company don't seem to know what their doing.

Many thanks, Phil.