mains water inlet pipe reduced from 22mm to 15mm before stopcock/water meter

[layman1]

hi, please excuse any ignorance on my part that this question may throw up.

my lead metal mains pipe enters the house and looks a respectable 3/4" outside diameter but within 6 inches it is reduced to 15mm copper to pass though the stopcock. this then passes through the water meter using 15mm connections and is then split off all over the place all in 15mm pipework. from the nearest outlet (1 metre away) i get only 11 litres/min flow rate.

i'm looking to fit an unvented DHW tank and wondered if altering the stopcock and water meter to larger bore versions to take the full 22mm pipe all the way through to the unvented cylinder would be worth the expense? could the relatively short stretch of 15mm pipe through these two restrictions really be the cause of my relatively low flow rate? and could i get away with just connecting 22mm pipe to the outlet from the water meter and hope this would help increase flow rate for the unvented cylinder?

picture to help can be found here [DLMURL]https://lh3.googleusercontent.com/-bGyVuD-FM-Y/VaPAwjl4rNI/AAAAAAAAByE/ce6Z3bxwYDM/s800-Ic42/IMG_20150713_144344.jpg[/DLMURL]

look forward to the ridicule, sarcasm and hopefully the odd enlightening answer!

 

[jtsplumbing]

Old lead pipe looks larger than you think the inside will only be 1/2" (15mm) need to get rid of old water main, if you are having a unvented cylinder fitted you need to upgrade the water main from external stopcock into the house with 20mm blue poly (minimum) depending what you will be connecting to.