Potterton Puma 100e problem

Hi,     I've searched google and cannot find the problem I'm having.
The system is a Potterton Puma 100e and it is about 6 years old. The main control board has been replaced, but that was a while ago, and this problem is relativly new.
The central heating and hot water work fine, but the pump runs more or less continuously. I would guess that there is some problem with the pump overrun circuit, but would not know where to start diagnosing it.
Any help appreciated. Iain
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