Lots of useful advice and guidance on this site – worth a read before you wheel in the big guns
There are laws about late payment.
Your best weapon is staged payments, a clear contract and payment schedule and making sure your client understands you care about the financial side of your business as much as they do about theirs.
The threat of legal action is usually enough to encourage payment. If that fails try these Solicitors:
Quick, economical and by all accounts very effective.