Charge him interest under some statute. I think it's called the Late Payment of Commercial Debts Act. Google it and you'll get some good help web sites. I think the gist is that if an invoice from one business to another is not paid within the specified time (default of 30 days if not mentioned) then you are entitled to charge interest at a percentage above the average base rate for previous months. There is no requirement that you put on your invoice that you intend to invoke the Act either.