I woke up at 3am, lots of things running through my mind. Mainly work stuff. Then it just came to me that I hadn't actually checked to make sure I had the correct pair of wires on the low-tension side of the coils.
Strangely, the factory and Haynes manuals had no clue about which pair belong where. I just thought, what the hell, switch them around after work and give it a go. And go she did! Just like that. You could have knocked me down with a feather.
So the engine wouldn't run before because the timing was exactly 180 degrees out (i.e. approaching BDC instead of TDC) on all cylinders. The coils are wired in two pairs to fire every 360 degrees, which is towards compression on one cylinder of the pair at the same time as the other is on its exhaust stroke. Instead of that, I'd had it sparking towards the start of the exhaust stroke on one cylinder and the end of the induction stroke on the other.
It isn't quite the end of the rebuild. MOT and tax next, plus some shake-down rides without the fairing lowers so I can see and sort any problems if and when they develop.