Gas Pressure Loss on Boiler- can someone explain?

I have a Worcester Bosch 42CDi combi (6 months old). When installed, the engineer had to fit a new gas run from the meter to the boiler as it was being starved of gas. WB came round today to replace a broken drop down panel. The guy did some testing and found only 17mbar at the boiler, yet 20mbar at the gas meter. The new pipe run is 10m long total and starts in 25mm and then reduces down to 22mm somewhere in between. Should the whole run have been in 25mm? What are chances that the boiler has always been low on gas pressure or does gas pressure supply fluctuate greatly? Is a 3mbar drop over 10m normal? Want some guidance before I contact the installer.