Many thanks for all your replies.
Agile
I haven't even heard of a listening stick (I actually thought you were pulling my leg)
http://www.hydrohire.com/acatalog/leak-detection.html - £80 to hire ouch
Gasguru
or a stopcock key or pump pliers (had a go with normal pliers to no avail). I googled check valves and see what they are but don't know how to check it they are functioning correctly.
So you wanted me to to take the pressure at each outlet, when I was measuring the flowrate, with the other taps open?? to get the the corresponding pressure? Wish I had realised that then.
This is all getting MEGA confusing and measuring the flow was a RIGHT pain as I had to drag the washing machine out again which is like a baby elephant, and measuring the flow under the sink was basically like having a shower as the end of the dishwasher hose disappeared behind the units so I couldn't put that in the bucket, so I had to disconnect the hose, and measure the flow coming staight out the pipe, and try and direct it into the bucket with a plastic bag. Everywhere got soaked! It was spraying out like a jet! My husband had to run around doing the timings and turning the taps on and off as I couldn't let go of the bag. Outside, the first TWO buckets I used had splits in so leaked all over my feet when I picked them up. It was great!
Anyway, it's all cleared up now, and now I find I should have been measuring the pressure at the same time! I thought I cracked the pressure questions earlier today. I quit - I can't do that all again.
So Gasguru or Agile would you be so kind as to visit and you can discover the holy grail figures you seek. We are near Leatherhead, so close to London.
I really appreciate all the help and suggestions. But my one question seems be leading to 5 more, and then 10 more, and it's got a little complex for me with no plumbing training.
I don't understand why none of the 4 plumber who visited and quoted actually tried to measure the flow or pressure, when it is apparently such a crucial element of the equation. ...