I have the same problem. Every time the boiler is turned off at the mains or there is a power cut, after about a min or so, the pump runs, and it runs forever (the burners are not on in this time). During this time if i run a tap or turn on heating it works but afterwards the pump continues to run... forever.
However, I have discovered if a hot tap is running or if the heating is on (and thermostat set to on) before the boiler is turned on at the mains and then turned off a few seconds after the burners start then the pump will stop after its normal over run of a min or so and then stay off. They boiler will then function as normal.
I would still like to find a cause/proper fix to this as I am concerned if we have a power cut when out/away then it will run its pump forever until we return to "reset it".
I have tried resetting the service settings on the PCB to defaults but that makes no difference. I also tried turning off the syphon pre-fill cycle at startup (again in the service settings) that didn't help. I even had our gas engineer change the PCB at it's service and that also didn't make any difference and the engineer had no other ideas as everything else appearked ok at its service.
Anyone have any ideas?