We have just been through something similiar. We needed a cert. of compliance for a planning applic. that had been submitted by a previous owner. When we bought the house it wasn't an issue but as we tried to sell the house the purchasers solicitor needed it for the sale to go through. Our engineer had to check back through files at planning office, visit site and sign off that it was constructed per the planning permission. This was done within a week. The plans of house should be in storage or on micro-fiche possibly. This should not be a big problem just more expense!