replacement boiler conundrum?

I have a 26year old Vailllant sine combi boiler that to date has been very good but is now beyond economical repair. Im looking to replace it with either an other Vaillant or Worcester Bosche. Its for a 1 bedroom flat i rent out currently with tennants so want min disruption

The plumber that has been in recons i'll need to rip out the existing 15mm gas main as he states manfacturers require a 22 mm main .Is this true or is he looking to make a bit of profit on a gas main on what would otherwise be a very straightforwrd installation

Simon B
January 2011