My house is fully plumbed on high pressure water; ie no cold water storage
tank and an unvented HW cylinder. It provides a pretty flow of water at the
taps/shower, but the shower is definitely affected if another tap is run or
the dishwasher fires up. I'm shortly going to install a second shower to
cope with the demands of a growing family (ie, a 13-y-o daughter) and am
concerned that if both showers are run together (which would be the whole
idea), then neither will be satisfactory.

The plumber who fitted our unvented system a few years ago did express
concern about our incoming watermain, and said that we might need to renew
it if the pressure was inadequate. I can't remember what bore it is where
it runs underground 25 yards out to the road (but it was partly exposed at
the time, hence the plumber's remark), but what comes up through the kitchen
floor is just 15mm copper).

Anyway, I'm now considering biting the bullet and installing a 'proper' blue
MDPE pipe, which means digging up the drive and considerable expense/hassle,
and so not something to be undertaken lightly, and unless I'm 100% sure it's
going to make a decent difference to the water flow at the two shower heads.
So my question is, is there a way of finding out? How can I be certain that
the bottle-neck is caused by the length of water pipe under the drive, or
elsewhere (eg the yards of 15mm pipework inside the house, or whether the
water pressure out in the road is not that great in the first place?)

Thanks for any pointers
David