Potterton Performa 24 Keeps Starting Up?

Hi All
I have a Performa 24 it has been working until recently, had a problem with the gas valve. Plumber sorted that out. Then yesterday, I was running the hot water. turned off the tap, the boiler shut down as normal. Then kicked back in as if the tap had been turned on. Now it just keeps running unless I turn it off altogether. Called plumber but he is at a loss...Micro swithes are fine etc. Can it be the PCB?? Any help would be appreciated. Cheers Rob