As soon as they go over the agreed terms you are allowed by law to charge 8% p.a plus the bank of englands base (0.5%) rate so at the moment 8.5% interest. I have one at the moment that we do several properties for, 90 day payment terms, there at 105 days now. I am seriously considering letting all our larger commercial work go. Vowed when I started to stick to domestics and small commercial, wish I had, we are too busy as it is with these, and I dont need the hassle of chasing commercial payment.