I did scout the website a bit before taking the plunge (well, a low cost splash anyway) for some new medallions. I thought (dimly) that the various sites - Amazon, eBay etc - would list the hole diameter and the OD of the growler decal part. They do (although some can't make up their mind between 50 mm and 2"), so I measured, and ordered. Naturally I checked that my XK8's year was specified. And Lo! The new medallions are all nice and floppy, due to the third unasked-for measurement: reach. No one asked, no one specified, therefore how could one ever know?
The answer evidently lies in the fact that my car has 19" factory (Atlas) wheels!
Rather than hassle with returns (not to mention that I still don't know how to order the correct medallions - except at Jaguar, for Lord knows how much) I did some measuring of old and new; the new and found that soft poly string about 1/8th OD can be laid (one pass) across each of the four spring clamps, zig-zag fashion in the cavity, and tied off at the back. This nicely shortens the reach of the retainer springs but for this to work, the string must be laying against the wheel-facing part of the plastic spring finger. Then it is necessary to run a piece of thin aluminum duct tape from just ahead of the string, up and over the spring finger and an 1/8" or so behind. This, along with some Vaseline on the foil and wheel opening, will keep the wheel opening from pushing the string away from the plastic spring fingers whilst the medallion is pushed home. In other words, the string and foil tape are between the spring retainers and the metal of the wheel opening.
BTW, it did occur to me that there might have been some ring-like retainer that would have to be applied from behind and with the wheel off, but the old - and presumably original - medallions fit perfectly, leaving no room behind for retainers of any kind.