As I didn't get a large response regarding the question on water pumps, I thought I would publish a new thread, to relay what I did eventually find out. This may all be well known, but it wasn't well known to me, and it sure as heck wasn't known - well, or otherwise - to the vendor RockBottomAutoParts.
There was an original design pump and then a redesigned pump, as of 2003. In between times, the original pumps had problems with black nylon impellers, but this issue was addressed by a TSB: The nylon was replaced with a different and successful plastic impeller. However, the latter was black, like the original nylon, and there was confusion. Jag's TSB was aimed at preventing wholesale change-outs of pumps based on the impeller color alone, and presumably this led to the equally successful white version of the plastic impeller of the later versions. My water pump change was pre-emptive, based solely on age and 60k miles, but the white impeller is still perfect.
I don't know whether this successful plastic overlapped with the pump redesign. Subsequently, the aftermarket seems full of original design pumps (particularly as this pump is used on many vehicles by many manufacturers), but often with metal impellers. These pumps can be had for well under $50. Or, you can buy from Jag.

Moving on: the 2003 -2005 pump is a different design, and is made in two parts. The 'bottom' part (nearest the engine) also functions as a spacer, so that the pump height remains as for the earlier cars. There are different shaped gaskets between the engine and spacer 'base' and also between the pump and the spacer. Without the spacer the pulley would not line up. Nor can the early style pump be used with the later-style's spacer, for the same reason. The old style will fit without the spacer, in other words, and the engine / pump gasket is correct. I saw reference to a .4 mm gasket and a 1.5 mm gasket. The thinner may be the paper gasket between pump and spacer, The 1.5 mm is typically aluminum, with formed rubber around the two side ports.
Having been shipped the wrong, early style, pump, and with O-ring questions unanswered and days ticking by, I opted to install the new original style pump. To do this, neither the spacer, nor the two additional bolts that hold the spacer to the pump, are used. one simply bolts the pump to the engine, with the supplied gasket.
There is one outstanding question: The 2003 model revisions included an a bigger engine and 10 more BHP. Was the redesign of the pump aimed at a greater water flow? I have tested my installation and the temp gauge sits nicely in the center, but then again it isn't Summer in Georgia yet.

 

One thing that hasn't changed over the years is the XK* temperature gauge. Unless you have either WhiteXKR's 'RealGauge', or some other means of reliably measuring engine temperature, then the OE ''idiot' gauge will sit comfortably in the middle until just before the world (and your motor) stops turning. It will not report an overheat condition until it's likely too late.

Mike