I've replaced the ECU and reset all the codes (for example the "power lost"!)
Started it up and it still misses on one cylinder. Checked the codes and it's cylinder 3 ignition.
(I think it is anyway, does anyone have a "code book" for the cheap code readers that's got European models in ? Mine says "For an EU 320 take the nearest US equivalent which would be a 318 because they've both got 4 cylinders". Oh, really, they must have put 2 spare cylinders in my engine bay for some reason then....)
So, as it's got a new coil, just maybe the coil was DOA, so swap it to cyl2.
Reset all the codes again. Restart engine. Still cyl3 ignition fault code.
So, check the bentley manual.
Check from pin 24 on the ECU cable connector all the way through to the connector going in the back of the coil. No resistance, not broken.
Turn ignition on. Check for voltage on the relevant pin of the coil connector as in manual, voltage present.
Check the earth lead. OK
Check the resistance of the coil. OK
So, now I've got a coil with power and a connection to the ECU. And it's still complaining.
At this point I'm pretty much out of ideas, I did think maybe there was a wiring short somewhere. Checked from the earth pin on the connector to the big battery + terminal under the bonnet and got a very low resistance. Thought that was it and checked the other connectors, they were the same. So I'm guessing I was just measuring the resistance of the interior lights or something..
So, either I've got a bad ECU (it is off eBay... but it seems unlikely that it would have exactly the same fault as my original unless anyone can tell me that it's a known fault with cylinder 3) or there is something REALLY freaking weird going on!!!!