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If the loft has been done properly, and it's been done relatively recently, I'd be very, very surprised if it wasn't inspected by building control.
I personally wouldn't accept the risk of buying a house without the building consent; you could make it a condition of completion that the thing is signed off which would only make it their problem I guess but would allow you to proceed along the way to completion.
There's no sense in you running around trying to get their signoff when the sale could still fall through for some other reason - all you'll have done is a massive favour for them for their next sale and removed your own leverage in the deal.
Good to get some different opinions, thanks. I guess it boils down to whether we accept the risk or not... Why can't people just do things properly?!