A build like this would normally have a retention built in.
There is a retention (enough to do a new boiler and cylinder install, I think). Despite aspersions cast, at this point I'm more concerned about it being right and safe than about the money, but if it comes to a new install the money will be extracted from the builder's pot not from mine.
Sadly, I don't think I can find a way to charge him for the weekend I spent reading installation manuals and Approved Document G, and other assorted plumbing and heating references. Though on the bright side, learning stuff is interesting in its own right.
The cost that I'm bearing is that I'm keeping 'my' professional opinion person paid by me - the advantages of maintaining a completely transparent situation about who his client is and really is makes that worthwhile. An hour or two of his time is easily worth the peace of mind - and it's less than we've spent on external light fittings (for example). Once all the dust has settled I might haggle with the builder over the final retention release, but that's a long way in the future.