I think the electrical problem with the ECU is solved. The first problem was that - yes - I'd misinterpreted the wiring diagram, my fault, but in my defence the power wire to the ECU is red & in every car I've ever worked on red is permanent live, but hey-ho. The next problemette was that I'd connected the wire to the master switch in the engine bay, so had to dismantle that to get it out. Of course I now have a LARGE stock of wire to choose from when looking for a new piece to connect it up with & found just the thing.
While wandering round the garage I spied a couple of unused fuse boxes & decided the best course of action was instead of tacking wires onto some other existing branch, fit a new fuse box. The existing relay that controls the injectors, Idle Speed Control Valve etc had an in-line fuse - I don't like in-line fuses, I worry I won't think to look for it at the road side when the car's died. So with a little re-wiring I re-routed the supply to those things through the new fuse box at one end & installed a switched live supply at the other, each with an unused "way" for future growth & wired it all back up. It worked! -OK, I think it worked. Pretty certain really.
Next job, sort out the alternator pulley. On the way to the Rolling Road there had been a nasty grinding / rotating sound when pulling away in first, on the way home this had vanished - but was replaced by the shrieking of a drive belt. Chris-the-mapper had pointed out that the alternator pulley was rubbing the engine sub-frame & now I came to look at is more closely the belt was now loose - the alternator had been pushed back as the engine moved forward under load.
I ordered a shorter belt, hopefully - job done.
But it is nice to be working with all the old sheet metal tools again.