I've seen how long it takes to adjust the valves on an XK engine. First you measure the valve gaps and write each one down for all 12 tappets. If on this first step any are tight you'll have to redo them. Next you have to remove each cam shaft, which is a somewhat involved procedure. Then you pull the cam followers and reshim them using your written figures (or remove some shims for a remeasurement - if they were tight you don't know -how- tight). Then reinstall the cams and recheck the gaps.

On a V12 the labor is exactly doubled. And all the V12 ever did for a Jag was make it drink more gas - the extra weight almost exactly made up for the extra power.