Water pressure dropping after a couple of mins - no idea why | Plumbers Talk | Plumbing Advice | Plumbing Forum - Professional and DIY Plumbers and Bathroom Fitters Forum

646 sqft apartment
1 ensuite bathroom
1 large(r) bathroom
150L(ish) boiler/water tank
Grundfos Amazon STP-2.0 B ( AP030) (i think)

00:00 Water pressure upon opening is fine with hot water all good
01:12 You can hear the water pressure slowly dropping
01:20 Water pressure just dies
[switch off shower] [you can hear water refilling up the tank]
03:00 Switch back on and it resumes
04:40 Pressure starts to die again.

The ensuite bathroom next door runs on an electric shower unit - and strangely doesn't seem to have problems (as far as i can tell)

Had one plumber come in who immediately suggested replacing everything with a PulsaCoil Eco Thermal Store
PCS180ECO (around £880) plus labour (around £400)

This is not a +£1k problem in my opinion!

Looking instead to replace/upgrade with a Gledhill Stainless Lite Plus Open Vented Direct Cylinder 150L at a more reasonable £350, which apparently is mains-pressure driven (which may dispense the use of the existing pump?)

Any thoughts/opinions would be greatly appreciated!!

Thanks!