Please don't be put off by this one problem. Houses which are more than 10 years old are not sold with NHBC certificates, and there are millions of them sold every year. However, if you have a full structural survey done this will highlight any problems with the construction and/or condition of the house. It also may be worth contacting NHBC or Zurich to see if they could provide cover but I can't see what that would offer. By the time 2 years have passed an NHBC or Zurich new house certificate is practically worthless anyway - believe me I know, I have tried to make claims and I found them a waste of time. Even within the 2 years, all they do is refer you back to the builder. I would also check with your mortgage lender to see what they would accept as an alternative (if any is needed).
So don't abandon this purchase on the back of a fairly worthless and unnecessary piece of paper.