Stripped the loom as far as the 12v splits into 3 but all looks okay.
If I measure the rear coil using the +/- on the connector I get approx 6v but if I use the +ve and any -ve on the other coil plugs (or any earth point) I get 12v.
Looks like the fault might be to do with that coil's -ve supply? Any idea where that goes?