I’m not sure whether to be happy or just confused. I fiddled with all the plug leads I could get easy access to, poked at the injectors, and waggled any sensor leads that had LHS/RHS pairs. I then tried a very basic CO monitor that I use to use for setting the carbs on an old VW I had. I wasn’t expecting to see the same absolute values that a pro machine would, but I was surprised that I couldn’t see any relative difference in the two banks. Which seemed odd as there was an order of magnitude difference at the MOT test. Without much else to work with, I took it into a local shop with Jag experience and asked them to figure it out.
They can’t find a fault, both banks now running within spec. I had previously taken a plug out to see what type it was, I guess it may not have got the plug lead on correctly, or maybe it was one of the sensors, or maybe … dunno. Happy it is fixed, frustrated that I don’t know why.