Verdict: loose connector. Looked like it was plugged in and locked shut but wasn't. Had an expert look at it (ZBeast from Perth4x4) and when he really pulled on the connector it came open. Pushed it back together and away you go.
Odd that the computer didn't report anything wrong but there you go.
Obviously I'd just stretched it while putting the new batteries in and it wasn't quite making contact. I'd checked the connectors but wasn't rough enough with them.
You learn something every day.
B.