Thanks Thermo.
We in the UK have not been allowed bonnet mascots for many many years. Nanny State!
Perhaps we are more prone to be scooping up pedestrians than you lot so our UK spec X types were delivered naked. I think all UK Jags pre 1976 did have them and these older cars are allowed to keep them.

 

Thanks for the useful comments on this thread which give me plenty of ideas of how to fix this problem. I have a 2004 XJ 3.2 V6 on which the retaining wire gave up but I still have the leaper, spring and retainer so almost good to go. However, the base plate just won't budge. As you can see in attached picture, the nylon nut came off easily enough, but it's literally stuck to the bonnet, perhaps the result of 16 years of heat and humidity in Hong Kong. I prefer not to force it and run the risk of taking a lump of paint from the bonnet. Any suggestions? I notice also that the threaded section has two flat sides. Is this designed to take some special tool? Thanks for your time

 

Flingel, if you are worried about taking up the paint, then get yourself some dental floss. You can slide it between the emblem and the body of the car. From there, you can pull the dental floss back and forth to cut your way through the rubber gasket that is there. Work your way around the emblem and this should allow you to remove the emblem base without any damage to the body or paint.

 

Thermo,
Thank you for your suggestion. I was going to ask if floss should be waxed or unwaxed...
Floss certainly helped but it was actually the positioning lug to the rear of the base plate that was proving obstinate and in the end required a bit of judicious prizing with an old plastic golf handicap card and a narrow paint scraper as per attached picture.