Thaks to all those who posted ideas for a fix
the problem turned out to be flame rectification.
The pilot flame was not big enough to carry the voltage input to the PCB which inturn gives the supply to open the main gas valve, a bit of jetting and all is well.
Can anyone help please. My potterton profile will not fire up. The problem is I am getting no supply from the PCB to the main gas valve, Pump runs- Fan runs - spark ignights the pilot - pilot stays alight. I have checked the function of the main gas valve by piggybacking suply from the pilot...