Thanks guys, I'll pop up a post on rsowners too!
I've had another fiddle this evening, specifically the earth around wiper mount. I bridged this direct to battery with no change

I guess this suggests the earth here is/was ok
I also tried disconnecting fuel black ecu, and it doesn't change anything - would've thought this would make things worse! Also tried a spare fuel ecu with again no change.
Can anyone tell me how many/or where earths should be in engine bay? maybe the fuel ecu isn't powered properly if its the same without it connected? On my engine lift hook (close to wiper motor) I've got a treaded hole with nothing in it - could this be an additional engine earth point i'm missing?
Another query, not sure if connected, is I have 3 empty small femalespade connectors with nothing obvious to plug in. One at the wiper mount, and a pair an inch off the main ecu - does anyone know what these are for? (will try to attach pics of these)
Thanks again guys, Andy