I put this on my invoices :
Payment to be made within 30 days of receipt of this invoice.
We reserve the right to claim statutory interest at 8% above the Bank of England base rate for late payment in accordance with the Late Payment of Commercial Debts (Interest) Act 1998.
(also you can chage an extra admin fee of £70 up to a 10K invoice and £100 over that)
Doesn't necessarily always work and I've had a few arguments but at least it gives you a legal foothold should you need it.