My time has come to address the dreaded valley hose, thankfully on the naturally aspirated engine it shouldn't be as bad to get to. However lack of time right now means I'm punting to the mechanic for this one. While waiting for his confirmation, I started looking into the knock sensors figuring I might as well have him replace those as well. However I'm finding two distinct model numbers matching the XJ8: AJ89189 (two senors per connector) and AJ85676 (one sensor per connector).

RockAuto shows the two headed one when filtering for a 2009 XJ8, FCP says the one headed is likely not compatible, and SNGBarratt shows both as compatible with the X350. Adding to the confusion is that the workshop manual diagram shows one head per connector, seemingly contradicting RockAuto. What gives? Did the later models add an extra sensor?

From VIN G00422 onward (2006-on) it should be AJ89189 which is two knock sensors going to one 4-pin connector. That is what is in my 2006 when I replaced mine.

Thank you for confirming, interesting to see that some parts were changed as the generation progressed.

2003 (EU), '04, and '05 were the "early design." 2006 saw significant changes to modules, brake system, etc. In the parts catalog there's almost always a VIN break at the G00x which represents the change to 2006-onward models. The 2008-09 should only differ in the facelifted front and rear, otherwise they were the same as the 06-07 under the skin.

There is a Jaguar documents floating around which explains all the changes made in 2006.