Boiler Mate 2000 - Calling for heat even when reached correct temperature.

.

I noticed on my system the same behaviour when temperature in tank is low. I have unusual wiring with dual timer. One for heating and another for tank, so tank is not always heated to optimal temperature. When it's temperature is low and I open hot water then pump starts but never stops. The only way to stop it is to switch off/on BM or switch on timer to heat up tank.
In your case the problem can be in thermostat on boiler, is it on maximum? When pump stuck try to check tank temperature. I can't remember what is optimal value but you can find it in user manual. Or may be tank not heated properly because boiler overheats and stops often, or pipe between boiler and tank clogged.