telling the client that you will go to the small claims court always helps... don't get me wrong though..
Not in my experience, though depends on why they haven't paid. If someone doesn't want to pay you, they won't.
More and more firms instruct their accounts depts not to pay until a final warning comes in. That might net them 60 days or more interest on the cash sitting in their account. Spread that across all their creditors and you may be talking a hefty sum. That's just "good business practice" in this day and age. Alas the OP here is to blame*, for sloppy accounting, he's given them 365 days free interest on their debt, more fool him! He should know who paid what, when, where and to whom. Requesting the cheque number which had allegedly been paid would allow a trace to be put on it.
The best thing to do is to make sure your trading terms are on the invoice, after 30 days send a reminder. After 45 days send a recorded delivery letter threatening court action, with a dummy County Court claims form filled out with all charges and fees. That usually does the trick.
Actually putting the claim in isn't so clever, the firm might just wait until the very last moment before the judgement is defaulted and then pay up. You are then out of pocket for the court fee.
If they are not paying because they are totally skint, getting a judgement will make no difference, you might be in a long list of creditors, might be the final straw which tips them over the edge into liquidation. Your money is as good as gone then.
* Not meant to be an attack on the OP, I realise that for a small amount, it's more hassle than it's worth to chase up the unpaid bill. I know I generally have more important things to do than spend time chasing up
