Thanks everyone for your replies.

Andy, so if my 22mm pipe drops to 15mm within 1m of the boiler I may be
ok - I'm thinking the pipe change may be in the wall as opposed to
under the floor - any ideas what the standard practice is, is it even
permissible to lay concreate on a 15mm pipe? Can I get a Corgi
engineer to measure the pressure to see if the 15mm pipe will suffice?