Good question! If say the job costs £500 and will be completed in 4 days, offer £125. 125 x 4 = 500. Also if possible, look to make sure the job IS being completed each day.
Jane S to contractor: "Listen, I don't care how many peepholes, hidden cameras and hidden microphones this Randy D character paid you to install here, but this is my house, I pay the bills and it's my word you go by, not his. You do know that he's an escaped mental patient, don't you? I'm willing to bet that he paid you with counterfeit money, too." ~