The mains pressure in the sealed system seems quite high, and I'm
guessing that I need to either reduce it (fit a PRV) or
fit the 'arrestor(s)'.

The sealed system has nothing to do with it. The pressure is set by opening
the valve until the pressure is as required. Whilst the US happily leave
theirs connected to the mains via a pressure reducing valve, us UK types are
sensibly precluded from doing so.

However, assuming you have a water hammer part in your water supply section
(hot or cold) rather than the primary heating circuit, then indeed, both a
PRV and a shock arrestor can help. However, I would measure the static
pressure and only fit a PRV if the pressure is above around 3 or 4 bar.
Otherwise, shock arrestors it is (or even as well, in the case of a PRV).

You want to avoid using a PRV if the pressure is lower, as it may reduce the
flow rate.

If I fit the 'arrestor' do I really need one for each tap where the
cold water is? Or can I fit it in a place where all taps/showers will
benefit?

The closer to the tap they are, the more effective they will be. You may
find that you get away with one in the bathroom covering all outlets.
However, if you have a serious problem, you may need one per fitting, fitted
closely.

As for fitting one in the kitchen to cover the bathroom, you could give it a
go, but you might be disappointed (or might not).

If I fit the PRV, I have no idea where to fit this, and help here
would be greatly appreciated.

Fit it in the rising mains, preferably after the garden tap (and maybe
kitchen tap), but before any bathrooms, laundry appliances and water heating
appliances (i.e. combi boilers/heat banks).

Christian.