I've recently replaced my spark plugs, and I noticed that the boots kind of sit on top of the plug. Are they supposed to fit snug on the tip of the plug?

 

In my experience (put on and taken my boots off many times chasing a misfire) they were a pretty snug fit, producing a bottle cork like pop when removed.

Hence, if they are a sloppy fit I'd consider either looking for some 2nd hand replacements from a breaker (I got a full set of coils for the equivalent of ~$80 in good nick of a low mile car [worth remembering XJ & XK use the same parts so doubles your chances]) or apply some dielectric grease to either the plugs or inside of the boots to get a good tight fit.

I'm pretty sure they are supposed to be a good fit to keep moisture out of the system and stop them arcing to the block.

Hope this helps

Alex