don't know if anyone ever measure the gas pressure with the boiler running. Having said that, the pipes were new 14 years ago, this was a new conversation when I bought it from a housing association, so I would guess they stuck to the regulations but spent not a penny more than they needed to.
don't know to be honest where the pipe reduces to 15 mm.
You would hope. But my experience of boiler installation in London conversions is not a good one in this respect.
But if the boiler is working it is a 30 second test.
Surprised no one has done it though.