Thanks Aerobod for the suggestion. In the end I went for broke and took every connection apart on the harness and used contact cleaner and dielectric grease to clean everything before reassembling, I mean EVERYTHING! Throttle body sensor which I took apart and used PCB cleaner that I had laying around, pedal connector, all of the plugs that I could find, ECU out and cleaned the contacts, the fuse box and relays all pulled out, cleaned and greased. I must have spent 2 hours, some of the plugs were quite tricky positions but I took my time, ensuring the connectors had a good soaking and dried before a light smear of grease before reconnecting. Finally connected the battery, small payer to the car gods and turned on the ignition. The engine warning light came on, the scanner was telling me the same error message. I had just given up at that point until I remembered that I hadn't tried clearing the codes. Clear all, rescan, no error codes I would definitely recommend cleaning connectors and getting a good dielectric grease, even for people building new cars, my terminals looked clean, not tarnished or anything that would suggest the problem could be down to bad connection but yet that is what seemed to be the issue.