Thanks for the replies everyone. The builder is adamant that the warranty value is correct and that the issue is with the lender. His view is to get a new lender. My issue is that, whilst we have an agreement in principle with a new lender, we are likely to hit this problem if we ever wanted to sell within the first 10 years. I just can't understand why the certificate value can't be upped? We've obviously invested quite a bit already and have had discussions about walking away from this purchase with the builder.
Does £150,000 for a six bedroomed, three storey house (approx 2,800 square feet) seem excessivley low to everyone? I'm ashamed to admit that I honestly have no idea.