Ok, I just went out and had a try - it seems I had already swapped K18 back when trying the other ECU, so unfortunately that's not the problem.
On K18 I also tried manually shorting 30 to 87(and 87a) - so forcibly powering everything up - Still no luck, but there was a bit of a spark when connecting the wire and I noticed the wire becoming warm - so something was drawing quite a lot of power. Is that normal? At least it shows the fuse isn't blown though.
It could well be dodgy fuel injectors (they don't look in good condition, and the fuel rail they came out of was rusty inside) - but even so, I stuck 2 LEDs (with a resistor, and both ways around to detect any orientation) in one of the fuel injector holes and didn't get any light at all when turning over - which makes me think it could be electrical.
I powered up the injectors and can hear them clicking, so I'd be surprised if I didn't get something out of them (even it was just a dribble).
... but I do have a working fuel pump with fuel pressure, and a spark - which presumably rules out the crank sensor?
One other thing that could be an issue - there was a section of the wiring harness that had 4 (i think?) ring terminals on it. 2 of them were brown and 2 were brown in black sleeving. Because they were all brown I'd assumed they all went to ground, but is that right? Should some of them have gone onto the + terminal of the starter motor?