Hi Jeanette
I use the info on the site here....
http://www.payontime.co.uk/
there's a calculator that I use there to work out interest.
If they are still owing money since last august then I'd say it's well past the time to be getting serious with them. Some things you could try....
Certainly cut off any more credit to these customers.
If it falls within the Small Claims amount then I fill out a claim form and send a copy to the debtor (before sending the form off to the Court) which makes it clear that we're serious.
Keep ringing them up and hassle them.
You could also visit the customer. Its much more difficult for them face to face to make excuses.
Out of all these, sending the Claim form to them usually works for us.
As an aside, do you credit check the customers before letting them have credit. You're probably in a different business to us but if we are at all unsure we ask for money up front, or at least a percentage. We do this for most new clients.
Good luck.