I have just read the book and it says that both the Cold Water Inlet and DHW Outlets are 15mm on the Boiler. So, should I pipe in 15mm or use an adaptor to go to 22mm??

Obviously the mains enters the house in 22mm pipe, is there any benefit to piping to the boiler in 22mm? Its obviously alot harder to lose 22mm pipes in the wall, so I only want to use it where it is beneficial.