I did strip the connector down (the black one too), it was quite full! Drained it and cleaned it and then cleaned it again, then washed the wire loom and basically, got all as clean as I could. I didn't open the ECU as I didn't want to damage it. I have a replacement harness on order and will repeat cleaning and check if any oil drains out of the ECU.
I haven't been anywhere (with the now cleaned connector) to give the engine a good test, but on a couple of short blasts around the roads here it does seem to have proper grunt again. Will have to get it warmed and give it a longer run under load and see how it goes. Crazy if this plug is the cause of power (and hair) loss! :crazy: