Quoted from Portsmouth Water..
"We will maintain a minimum pressure of water in a communication pipe
serving premises supplied with water, of 7 metres static head."

That is about 0.7 bar.

Is the 9 litres a min at 1 bar the national limit or does it differ?

Clearly it differs!

above for the Quote. I surpose that they would have to fit a larger pipe
to
supply our 3 flats with the desired flow and pressure !!!! For free if its
below the limits..

The replacement pipe will not affect the genuinely static pressure. However,
with such a low pressure from the street, you need a particularly large
supply pipe to get a good flow rate. Think 32mm MDPE.

Christian.