Use the Late Payment of Commerical Debts (Interest) Act 2002 - you can send him a statement of account listing:

- the amount he owes
- a penalty for being late - £40 up to £1000, £75 £1000-£5000, something else for more but cant remember how much
- interest at 8% + reference period base rate (currently works out to about 13.5% IIRC) annual (non-cumulative)

I normally say that if they settle within 7 days the penalty and interest will be waived and the original invoice value accepted in full setttlement....

Chris