I have recently had a flat roof re-covered by a roofing contractor. The roofing system is Rubberbond EPDM. The contractor supplied a detailed written estimate but this made no mention of any guarantee, either for the contractor's work, or the materials used, although it was stated that the system had an expected minimum 40-year life. Before placing an order I phoned the contractor and was told that a 20-year guarantee would apply. I have just been invoiced for the work done, but they have not supplied any written guarantee for the work done. Should I insist on a written guarantee before I pay for this work? The company is a reputable local firm and members of all the relevant trade federations.