TRITON OPAL Wanter cuts off on HIGH setting?

Hi - I have a triton opal shower unit a good few years old (was here when I moved in) that has started to cut off the water flow on a high setting (two flames). When I switch to medium (two blue flames) for cold water or low (one red flame) for warm these work fine. I have opened the unit to see what happens and have noticed that one of the heater switches works fine on its own when both need to be on when the HOT (Two flames) setting is selected heater switch B (the top one on the PCB) sparks and disconnects and everything resets - this happens as soon as you select the HOT setting on the control board. I think it seems to point to a PCB fault but not sure if anyone out there has had the same problem. I have cover for everything else and assumed the shower was included but my mistake was I didn't check!
one more thing I noticed was the ribbon cable from the control unit to the PCB is slightly burned. I suspect this would cause the control unit to malfunction though rather than cause a specific problem?
Any help would be much appreciated!