Hi folks. I'd appreciate some opinions.

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.

Now that our loft conversion is finished, the water pressure up there
is pretty poor, and downright unuseable if someone has a cold/hot tap
on downstairs.

Unfortunately the plumber who put the original Megaflo system in has
left now (went to Poland), so I can't ask him what he might have done
to cause the reduction in pressure. We had another plumber come in and
check it, and he says it's because of the 15mm copper piping we have
from the mains (and wants us to spend a couple thousand upgrading
that). That doesn't make sense to me because the pressure /used/ to be
great for a couple of weeks.

I've checked the mains stop cock in the house and that's open fully,
and I'm out of ideas now, other than consulting yet another plumber.

Could there have been something else during the construction that may
have affected the water flow? Or what else can I try to find the
bottleneck?

Thanks a lot,
James