Since last post, fault has occurred every day except one. Same symptoms.
Today I took car to Pete Twomley, a one-man band at Formula 1 Auto Electrics in Bournemouth (as recommended by Triton).
I've actually got three diagnostic ports. One on the engine (minus TE1 connection), a circular one under the dashboard which has various connections including E1 and TE1 BUT has no wires connected to it, and a third, true OBDII port right next to the boot release lever(see pic below). After no-go with the engine port, he successfully obtained these two codes...
P1120 Throttle / Pedal Position Sensor / Switch A Circuit Malfunction
P1121 Throttle / Pedal Position Sensor / Switch A Circuit Range / Performance Problem
He seemed happy due to the Toyota specific prefix 1. The last three digits tie-in with the (SAE J2012) OBDII codes and fault descriptions that Alex gave me....they also re-affirm my garages diag. last year.
Genuine or not, they certainly could apply to my specific problem. He cleared the ECU so that he can re-read any error code that comes up in the next few days.