How does it work with materials on site, as part of the builders regular installments? We've got a contract, a fixed price, and at each valuation the builder and I agree a % that he's completed and we pay that. Only thing is that materials on site are on top but they're actually supposed to be part of the original price. The accountant at the building company told me not to worry, and those costs will disappear by the final bill. I don't get it. Am I missing something really obvious?!