And Elvis was sitting next to Nick Evans in the spaceship, which I
thought was kinda weird, but then they turned to me and said:

The brass plugs are there as part of the manufacturing process. They mark
the places where the supports for the cores go in the casting mould.
Later, they are drilled out and brass plugs fitted to blank them off.

The plugs can sometimes loosen if the head is seriously overheated and
warped. Occasionally, they can become receded into the head a little way
if the head gasket leaks combustion gas past the firewall ring.

Replacing them is a moderately tricky engineering job (to do it properly,
they should be accurately machined to a precise oversize, and shrink-
fitted after heating the head), so the charge of ?106 including the skim
isn't unreasonable.

Seeing as second-hand A-series are common as muck though, I'm surprised
that the engineer even bothered. Unless you have an exotic modified head
that it is worth spending money to save, I'd expect most engineers would
reccommend scrapping it and acquiring a sound head from a breaker.