All much appreciated!
Ah...double ouch. Think this is one I will need to research and do most of the labour myself then, saving the actual boiler connection to someone registered.
Will need to check how the underfloor heating downstairs is plumbed but remain confused as to why at that point it wouldn't have been changed to a flow/return system. Can't imagine one pipe systems fair too well with underfloor heating. A separate combi boiler was installed in the extension which I have assumed covers the additional bathroom and ensuite but am now wondering whether this is what the modern underfloor heating is fed from and the basement system boiler solely covers the first floor single pipe system.