Since you are in Scotland and the invoice is overdue, you can legally add £40/£70/£100 penalty to your invoice (which one depends on the invoice amount) + charg

Since you are in Scotland and the invoice is overdue, you can legally add £40/£70/£100 penalty to your invoice (which one depends on the invoice amount) + charge 8.5% annual interest. These do not have to have been specified in your contract with the client as they come under provisions of the Late Payment of Commercial Debts Act.