Ecu voltage vs battery terminal voltage.
 

Apexi is reading about 0.3V less than a multimeter across the battery terminals both ignition powered before starting (less earthing current through ecu) and engine running (only tested at idle). Is this close to what I should expect or do i need to go chasing connectors/earths?

The main engine harness earth that bolts to the front plate behind accessories and front plate itself were cleaned up recently when i fitted aftermarket coils.

I'd say this is normal. I've noticed this on multiple PFC equipped cars with decent electrical systems.

Thanks

The ADC only has so many bits to store the voltage range within, which gets reduced even further when sent to FC-EDIT, so there's some loss in accuracy. I wouldn't worry about it unless its a significant drop.