My first attempt to discover if there are any ECU errors logged was thwarted by Yoko not having a 'Check Engine' light on the dash. A suitable LED and wiring has been made up to try again later.
Temporarily fitted the new head unit and it sounds fantastic. Can't do a final fit yet as I have to remove the tombstone, to fit some other bits first.
A second attempt to discover if there are any ECU errors showed that there no errors present. This doesn't help explain why the engine is running rich though.
Reading some more though, it appears the temperature sensor on the back of the engine can fail and sometimes not show up with an error code and it is also a common cause of issues.
I fitted all new belts in 2017 and the alternator and power steering belt has obviously stretched a bit. It has started to squeal slightly under load and felt a bit loose. There should be 9mm to 12mm of movement in the middle when correctly tensioned. This was easily and quickly adjusted though. No more squeals :-)
Also checked the oil level today and it was a bit low. After 208,000 km (129,245 miles!), Yoko does burn a bit of oil.