Yes, that's the sort of thing I'm after... it's just that the pipe isn't
(easily) accessible closer than... ooh, 4m to the toilet.

The closer they are to the valve, the better they work. However, they will
have some effect wherever they are installed. 4m may be pushing it.

It may well be worth examining installing within the cistern. Many modern
valve designs take much less room than traditional types. You might actually
manage to get the whole lot installed within.

Having said that, we also get bang when taps are closed too quickly,
particularly in the kitchen where they are 1/4 turn ceramic disc things,
so an arrestor somewhere more central in the pipework might not be a bad
idea.

Yes. Shock arrestors are very useful installed near to ceramic disc taps.
These can be as bad as modern toilet valves and washing machines, which are
all very rapid acting. It's a shame they can't seem to design a toilet valve
that shuts off fully over a period of a second. They only manage 1/100 of
second (so it hammers) or 100 seconds (so it squeaks into the night).

Christian.