Let's not get too far ahead of ourselves here!
As said above... Call and politely explain the situation, most "decent" customers are mortified if their cheques bounce.
The way you have to approach it initially is to broach the subject with them, bearing in mind that you need to remain discrete, friendly and on their side. If it's a genuine mistake and you get shirty you've lost a potential repeat customer and their referrals all because of an oversight on their part.
Only after the softly softly doesn't work should you get a bit more assertive. This reminds me, I've got a domestic customer with a bill outstanding from last October... time to polish up the knuckle dusters
