About that, I think it was properly tested but I will insist also when they come back. We checked that the gas supply from the gas meter is 22mm, so, without opening the floor, the guess was that it's 22mm all the way to the kitchen, then when the pipe "branch" to gas cooker and boiler (which should be about 1-2 meters from the boiler), it goes to 15mm.
You don't say what the total length of the gas pipe is, or the number of bends etc, or what is connected to it other than the boiler.
I wouldn't mind betting it should be 28mm back to the boiler from the cooker.
I would say there is about 6m from the meter to the boiler, with a small gas cooker right next to the boiler. I also have a small flame gas appliance (not really used) in my living room, about the same distnace from the meter, but no idea where it branches off.