Hi Martin
The law you refer to is:
The Late Payment of Commercial Debts Act 2002
This act allows you to do two things
1) You can charge interest at the annual rate of 12% for each day the your invoice is overdue
2) You can make a one off compensation charge against the over due invoice again if it is late by even one day
at the following rates.
Debt Size up to £999.99 you can charge a Fine of £40.00
Debt Size up to £9,999,99 fine £70.00
Debt size up to £10,000 and above fine is £100.00
Hope this helps
Trevor