Finally found the gumption to get into the workshop and look into why the Ed80 isn't firing up.
First hurdle....
Snapped this useless piece of trash trying to loosen the very first plug.....
Used the new spark-plug holder.... very handy indeed!
All 4 plugs spark fine:
(I've seen bigger sparks.... but they're definitely there)

(images cropped from videos so not best quality)
Stuck the tank back on to feed fuel, and still nothing.
I can hear the fuel pump priming the system, so that's good, and there's definitely fuel within the hoses (when I undo the hoses fuel drips out).
I start to ponder if there was something in the alarm wiring that would prevent the injectors firing - but I didn't remember there being anything like that, and I believe the injector wiring comes straight out of the Motronic control box, so I doubt the after-market alarm would be that intrusive..
Do some digging on UKGser (my current main source of info), and realise I can check if the injectors are firing by looking at the voltage on the electrical connections. So I do that, and yes, there's signals coming down the lines. Not sure exactly what the readings are (I think the battery in my multi-meter isn't great, and obviously the signals are varying). But there ARE signals.
So - it's hopefully not anything electrical.
I then pull each injector to see if there's anything wrong with the injectors....
nada.
Ah-Ha!
It's the injectors!
Ponder... ponder.. would both fail simultaneously?
hang on: I've got another bike here....
Rip an injector of Rock4 and check that on the Ed80. Still nothing .
Hmm - I KNOW that's a good injector, and the Ed80 injector fitted to Rock4 works fine (plenty of fuel spewing out).
There's no pressure in the injector manifolds... or rather .. very little pressure: the good injector "was" spewing out "some" fuel, but nowhere near as much as it should be.
There's 2 causes for low pressure in the injector manifold: The pressure regulator has failed (less likely), or: the hose in the tank between the pump and the mounting plate has split: VERY likely. That's a known problem on these bikes and it's generally an age-related issue, so very likely to be the case here.
So - I syphon out all the fuel from the tank (again) ; ptui.. I hate the taste of unleaded.... mind you 98RON is better than 95...
The fuel pump gubbins is all mounted to a plate on the underside of the tank, inside:
Last time I did one of these.... I'm not sure I got it oriented right when I re-fitted it.... so:
These little studs are tiny, and the lock-nuts tend to be very tight, so I dug out some penetrating spray:
After a close look, I realised that some of the studs had a little plastic collar around the exposed part of the stud: so I pulled them off with a pair of pliers and put the graphite spray to work overnight. Hopefully it'll make the nuts a tad looser, to be frank I'm terrified of snapping one of the studs, every time I do this job: they're only an 8mm nut, but it feels like you're undoing something bolted to the Firth of Forth bridge when they haven't been undone for a while, and afaik, these have NEVER been undone. I'll also drip some ACF50 on them today before having a go at them.
Quite happy to be making a little progress and I'm very happy that it seems to be a non-electrical issue that was causing the problem... I was worried that I may have missed a joint, or badly soldered a joint when I removed the alarm.