It's up near the top of the power board, slightly to the right of centre. There is a large capacitor plus the smaller one that I replaced. On my son's this was a light blue colour. The capacitor is used as a reactance to reduce the mains voltage to that required by the electronics.
Thanks seco services. Had tried re-soldering all the components to no avail. Being an electronics engineer I decided to trace the circuit out,(in the absence of a diagram) and fix it.
Been reading about the Triton Opal II low pressure faults. My son has one that did the same i.e. LP light on etc. Did all the usual checks, replaced the control board but still the same. Eventually traced the fault to a 100nf - X1 rated capacitor on the power board. This is part of the pressure...