Thank you all for the replies.
This is incredibly stressful and frustrating. I have to admit I'm shocked the issue was not raised earlier in the process.
I spoke to our sellers and their view is that it would be too difficult to get an NHBC certificate - which probably is true.
On the other hand if we re-apply with another lender we lose £2.5k in fees etc we already paid to our current lender.
If we proceed with another lender, could we then retrospectively apply for NHBC cert so if we have to sell we won't be facing the same issues?
Or is it only the builder of the property that can apply for it?