What boiler/central heating system do you have already?! Is it a combi? Or the older gravity fed one with back boiler etc, if there is lots of new pipe work needed and floorboards lifting etc then labour will be quite expensive, the site I'm managing at minute are fitting ideal boilers, plumbers say it comes with a 8yr warranty, but another plumber I know tends to fit Worcester Bosch boilers there not cheap tho,