The first thing I'd say is don't change your spark plugs in the gloom of an evening, you might miss things, like the crack down the side of the ignition coil and the melted blob hanging out the bottom...
Once the coil was replaced, it still wasn't firing on that cylinder.
(In fact it wasn't sparking OR injecting, except it tried to inject a couple of times on startup)
After much diagnostic, we decided it would have to be the ECU. Probably whatever melted the coil blew the transistor. Pulled the ECU and opened it up and found some water damage and a burnt spot on the plastic PCB cover where the connector pin for plug 3 solders to the PCB.
Probably the ECU is being clever enough to notice the lack of spark and cutting off fuel to save the engine/catalyst.
Big thanks to Darren at DCS Bristol! I can't recommend him because if you all go, I won't be able to get booked back in to his busy schedule :-)