Its an L6 ECU. It's just the voltage that's the problem. I investigated a bit yesterday and succeeded in immobilising the thing. Could be an earth where all the fuse tails and kill switch resistor are anchored.
The clutch is hydraulic but the running-in period seems quite normal with whoever I've asked. It was slipping like mad to start with and now holds better. New flywheel etc. I've only done about 100miles since rebuild but this voltage thing is the overlying issue. It conked out a couple of times and only ran when I attached a booster pack to the battery. It was a race battery and I've now got a new Bosch one.
I can see me stripping back the entire dash and tracing back all the wiring but I'm looking for something more obvious for now. At least I can rely on the alternator.