New system water pressure

Recently moved into a new build house with the following set up:

potterton Promax HE
Twin zones CH (Zone 1 = bottom floor and Zone = top two floors) via Siemens controls
Big Kingspan Range HE cylinder and two expansion tanks in the airing cupboard

This feeds a downstairs cloakroom, three showers on the first floor and one on the top.

The literature suggests I should be able to run multiple, simultaneous hot and cold water outlets. In reality this doesn't seem to be the case.

Running a bath for the children last week caused the top floor shower to come to an almost complete stop in terms of flow. Likewise when the washing machine kicks in, the overall pressure is reduced when your're running any other outlet.

The main feed comes in under the kitchen sink and has a PRV and in general the cold water pressure is good. Hot water pressure is also good when running one outlet

I expect the pressure to drop a bit when running multiple outlets, but does the above behaviour seem OK/terrible/the norm/ or is it time to talk to the builder?

Talk to builder time methinks!

Keith.

You can never be sure what the pressure / system is going to behave like otherwise.

Ask the builder if a water pressure test (both static and dynamic) was taken and the flow rate.

Water size MDPE blue pipe is under the sink, hopefully not 25mm. (32mm. would be the min in this case)??

Break tank and pump set come to mind.