Thanks Frank. I put a new power transistor in and it made no difference. I forgot to check the power unit before installing it. However took it out to test. When 1.5v applied across term 1 and 2, the resistance between term 2 and 3 drops but by about half. I'm hoping that means it's OK.

Just saw your post above re: the capacitor and filter. Will check those tomorrow - I did wonder about the capacitor... they fail fairly often and is usually the first suspect when a spark disappears from an engine with points (contacts)

Am also tracing the power from the ignition switch to the coil (wire 3-BW in the circuit diag). I still only get 8 to 9v at the coil, with or without the ECU and power transistor which should not be. I have now checked the back of the ignition switch - 12v, next just after connector C-66 - 12v. After connector C-66, wire 3-BW branches off and changes colour to RL via a multi purpose fuse to power the instrument binnacle (speedo pod). the main BW wire carries on to B-67 which is the last connector before the coil..... Apparently B means it's in the engine bay somewhere but I cant see a connector, only wiring loom. I'll check behind the battery tomorrow. Surely that connector is causing the voltage drop although I don't quite see how it would suddenly happen. Thanks for your help so far. Rgds