OK - An update on the situation. We've got a new plumber who has replaced the 15mm lead pipe which brings the water into the house with a 22mm plastic pipe. The flow has increased by a huge amount:
Garden tap with old 15mm mains pipe: 8 lpm Garden tap with new 22mm mains pipe: 17 lpm! Pressure remains unchanged at 1.8 bar
I've also run a hose upto the 1st floor (where the shower will be) and get around 14 lpm.
We have decided to go for a combi boiler (we've been recommended a Worcester Bosch 42CDi). The plumber has suggested that we also consider a Grundfos Home Booster as our pressure is pretty low. We want to avoid the Home Booster as it's big, a bit noisy, expensive, and something else to go wrong.
We've ruled out a standard system with cylinder & tanks due to space constraints.
So my question is this: Is our mains supply going to be good enough for the Worcester Bosch combi boiler without the booster? The Spec sheet says that it needs a 2.3 bar supply for the full DHW flow rate. We can supply 1.8 bar (or 1.5 bar going to the bathroom on the 1st floor) so it's around 2/3 of 2.3 bar, so we should get around 2/3 of the quoted DHW flow rates right?
Thanks!