Ah, the venerable Jaguar XK engine! This beautiful 6 cylinder double overhead cam engine was produced by Jaguar from 1949 all the way until 1992. While enhancements were made along the way, the basic engine design remained virtually unchanged throughout the entire production history.
Jaguar designed this engine to be easy to maintain and repair. This includes the design of the overhead camshaft mechanism, which allows minute adjustments to each cam for timing purposes, and, more importantly, allows both cams AND the entire head to be removed without having to disconnect the timing chain.
This is fortunate, because the camshaft adjusting shims are located underneath the tappet, which necessitates removal of the camshaft to change shims (and thus change cam-to-tappet clearance).
The video below is a tutorial covering the complete process of checking and adjusting valve clearances on any XK engine. The author makes no warranty as to the correctness of any of the procedures discussed in the video. Owners who endeavor to perform this procedure are urged to use the appropriate Jaguar factory manual as a guidance companion for details. (Note: Complete factory parts and procedures manuals are available on CD for most all classic Jaguar models at a nominal price from the Jaguar Heritage Organization: www.JaguarHeritage.org
The basic procedures outlined in the video, while specific to the XK engine, can be applied to most all overhead cam engines. However, some engines are designed with the adjustment shims sitting on TOP of the tappet, rather than underneath. Those designs usually allow the shims to be replaced without removing the camshaft(s).
Happy shade tree mechanic-ing!