After recently having a Megaflow mains pressure hot water tank fitted
and some additional radiators, I have noticed that the pressure in the
system needs topping up from the filling loop every 3 weeks or so.

There could be a small leak on one of the new joints. You may not see it, as
the water evaporates immediately on reaching the outside air. Go round with
a brillo pad (green scouring pad if plastic rather than copper) and clean
every joint well. After a few days/weeks, the leaking joint may show some
runs or staining, even if it is dry.

Also, the system may not have had inhibitor inserted when it was refilled
after the renovation. This may lead to corrosion, which produces hydrogen
that need frequent venting. This can be fixed by flushing the system and
refilling with inhibitor.

Christian.