JT wrote:

We've recently had a loft conversion done, and at the start of the
construction we had a main fed hot water system put in (Megaflo).
Immediately after that was put in, I noticed a fantastic increase in
the water pressure in the shower and all the taps around the house.
The shower was barely useable at full blast.

Strangely, a couple weeks later the pressure dropped again. It was
still pretty good but no where near as it was before, but it was
useable so I didn't think twice about it.

Well, if it was formerly OK and suddenly dropped, then surely you're
looking at a physical blockage somewhere, eg a bit of debris (left in
the pipework by the plumber) having detached itself and migrated to
somewhere critical. Is it just the loft shower that's got the problem?
The shower valve is likely to have a filter on the incoming pipes; can
you remove the valve and check that there's nothing in there?

David