My triton opal shower keeps going cold?

Hi
I have an triton opal shower that keeps going cold and the low pressure light comes on. this lasts for a few seconds and then heats up again.
I've checked the plunger is making contact with the micro cwitch on the power PCB. When i tested it with the front cover off I noticed when the low pressure light comes on there is a flash from the top coil.
Is this a faulty PCB? I've had the board off and all seems ok no loose connections etc.

Can anyone help?