JT wrote:
Thanks for all the replies!

Aiden, I cleaned the strainer, and also recharged the air volume in
the Megaflo, but that didn't seem to make much difference.

There is (should be) a pressure reducing valve on the Megaflo; it
should be factory set to deliver 3 bar outlet pressure. It won't do
this if the inlet pressure is less than 3 bar. Your problem is most
likely caused by a reduction in inlet pressure (i.e., reduced mains
pressure), or a defective or incorrectly set PRV. You'd need to get
hold of a pressure gauge to diagnose the problem.

If it had been a restriction, as I'd suggested, then the pressure would
creep up to the set PRV outlet pressure when there was no flow,
compressing the gas charge in the top of the heater. You'd then get an
initial 'burst' of flow at 'normal' high pressure, dropping off as the
air expanded and the flow rate from the mains declined. Sorry to
contradict myself.

You could check that neither of the 2 pressure relief valves are
discharging water. The discharge tundish (usually black plastic) should
be visible, so this should be immediately obvious.

If it's reduced mains pressure ( 3bar at the PRV inlet), you could
also contact the water supplier to enquire about the recent reduction
in pressure. They're not obliged to supply much pressure, so they may
have turned it down. It might also be caused by a large underground
leak and they'd want to know.