I beg to differ.
Before I go into details on how I solved the problem, here's a link to the Jag-Lovers XK Engine Tappet Retainer Stakedown FAQ. Read it through before you do anything else and come back here if you'd like an alternative view.
The procedure is simple: Instead of drilling holes where they are not needed and introducing lots of Big Pointy Metal Objects that could do a lot of harm if they ever worked their way loose, I simply inserted a screw beside each tappet guide holding it in place. When I say "I" I mean a friend of mine who knows his way around a machine shop (he owns and operates one). Trust me on this one: Recruiting help from a professional for this operation is not a defeat.
1. Overview of the engine with the LHS cam cover off.
You can clearly see one screw in place and the grease for
keeping drill shavings under control. You can also see the
paper stuffed into every opening in the head to prevent bits
and pieces (and shavings!) from falling into the engine.
Before doing anything else you should make sure that all the
tappet guides are firmly seated at the bottom of their seats
in the head. This is easy enough to measure, just compare all
six (or twelve if you're doing both sides).
2. Drilling the holes. Each hole was drilled at an angle
just slightly cutting into the tappet guide itself. The holes
where then tapped and the grease removed. I spent several hours
making sure I got every little bit of metal shaving out of there
before I put the cam cover back on.
3. One screw in place. You can see that the screw is sitting at
an angle, the bottom end cutting into the tappet guide (but not
through it!). You can also see that the head of the screw is
holding the lip of the tappet guide, securing it firmly (yellow
arrow). The screw is secured with Loctite.
You can also clearly see the damage done to the tappet guide by
its direct contact with the cam lobe, small chips are missing
around the inner edge (red arrows).
4. Job done and ready to put the cam cover back on. Note how
clean everything is, make sure you get all the shavings out!
Oh, and don't forget to remove the paper/cloth you stuffed into
the openings in the head...
After completing the job I immediately changed the oil and filter
without starting the engine first. I then drove the car for
one day and changed the oil and filter again just to make sure.
You should too.