Thanks for response, John

Sometimes the effect is sufficient to reduce the flow massively.
Mostly it's just an annoyance which scares the kids if it happens to them.

The screeching is intermittent and usually occurs once or twice for say a
minute in a 10 minute shower.
Yes, the old pump also screeched. The Aqualisa engineer replaced the pump
almost on the off chance that it might be the cause. Turning the shower off
for a few seconds and back on usually cures the screeching - temporarily at
least.

I have also swapped the cartridge (incorporating the flow control and
thermostat arrangement) thinking that maybe the stat was starving flow to
the pump somehow.

I have yet to try taking off the cover and running the shower with the guts
exposed. Being low voltage I guess this isn't too potentially hazardous.
As the feed pipes are back entry, I'm not sure that "waggling" these will be
possible.

Phil