A quotation is an exact indication of the cost of a job. It is given by the trader and is binding on both parties if accepted. If a job takes longer than expected you can't be asked to pay the difference. Conversely, if a job takes a shorter time or uses less material than first thought, you will still have to pay the quoted sum. The law applies just as much to professionals as it does to trades. It always pays to get a quote in writing.