I have the following scenerio; local authority watermain on road beside our house - this is a 3/4" main which feeds down gradient from a 4" main. This 3/4" main was put in 25 odd years ago for my parents house. We have a 1/2" feed off this 3/4" for our supply. Low pressure is not a problem for my parents and the stop cock is actually turned down to 1/4 of the power as the pressure comming down the hill is large. However, our pressure is far from adequate (and yes our stop cock is fully open).
I thought we might have a small leak on our service pipe, but having observed the meter over night when not in use I have now ruled this out.
Could our service pipe be silted up?? Does anyone have any possible explanations??
Before you do anything, open up all the stop cocks in the fully open position.
Slightly turning off a stop cock to try to "turn down the power", is like being slightly pregnant. There is no such thing as "turning down the power" on a mains stop cock.
A stop cock is either on or off there is no half measures. What a half turned stop cock gives you is the worse possible scenario. You stll get the full dangerous pressure of 120 psi, but you get miserable flow. So the stop cock at your parents must be made fully open, it is doing no good for you further down the line trying to get a better flow.
Also sounds like many a case where a house is situated on higher ground. It may take immense pressure to get the water up a hill to fight the "pressure head", but just pumping at a pressure peak enough to allow the water to dribble out the pipe at the top of the hill, where lower down the hill the water would be flying out of the pipe on the same pressure and pipe size.
Pipe length and elbow fittings etc, also causes flow drop on smaller pipes. (Theoretically a single half inch pipe feed is plenty large enough to supply enough water for ten family homes all week long, but not if all the homes all need a lot of water at the same time.)
If a half inch heavy duty pipe is run for a few hundred feet or more, plus a number of elbow fittings are plumbed in, then flow could drop for sure.
Either way, high pressure like 120 psi needs regulating down to 70 psi or under, and if there is no blockage in your pipe, and all stop cocks are fully open, then you will probably require a 1000 litre buffer tank and booster pump.
The 1000 litre tank can take all day to fill, even if your flow is under 1/10th what it should be, but the booster pump then gives you as much flow as you need from the 1000 litre reserve when you want it. (You may want to fit a loss of prime switch to the pump to protect it from running dry, if ever the tank empties. Most families of 4 use only 220 litres a day, so this is very, very unlikely)
If you pay water charges to your group scheme, see the Charter of the NFGWS, the National Federation of Group Water Schemes, they have to guarantee a minimum 22 psi water pressure or a minimum volume of supply. You can take your water scheme to court and not have to pay your water charges until they fix low flow issues, or get money back that you have previously paid, or get them to pay for a buffer tank and pump.