The water pump seems to be operating even when neither the hot water nor the central heating system is calling for heat. There are three levels marked on the pump and when either of them are selected the pump works except where the lever is just below the level II mark.
The pump is a GRUNDOS SELECTRIC , Type 15-50 about 4 years old.

Kenneth Stanton, October 2012

sounds like the end switch is stuck on a motorised zone valve, 2 port valves are common try tapping the valve and if the switch drops off pump will stop

higgy, October 2012

It's common in boiler pump keep going until cool down to set tenp

ltd, October 2012