Hi Ed,
If your problem is a straight-forward non-payment I can recommend [broken link removed]*, they focus on small-to-medium enterprise debts and use an automated reminder system. They charge 9% commission on a strictly no win, no fee basis.
For more complex issues, such a contractual dispute, I would recommend a solicitor rather than a debt collection service.
* Disclaimer : I have worked in a technical advisory capacity for this company