Potterton Netaheat - pump is on when system is off?

I have a Potterton Netaheat boiler - (not sure of the model number). This morning, although the system was turned off, the bathroom radiator - the first on the run from the boiler - was slightly warm.
It seems that the pump had been running for a short while. I know that this can happen just after the system turns off to dissipate excess heat in the boiler but have never experienced it when the system has been off overnight.
I'm wondering if the 'excess heat' switch in the boiler is faulty.

Any ideas?