Charging interest can be very dangerous, especially if you're NOT a credit provider. This will leave you in a more vulnerable position, if you did not charge the correct interest rates,as well as the terms.
I believe different industries have different interest rates, eg. micro lenders can charge up to 60% per annum. You're not a micro lender, yet you charge 120% per annum, this will unfortunately count against you.
The legalities are high in your case, and my advice is that you get in contact with a debt collector, as they will advise best, and they're way cheaper than just jumping to a lawyer. Most debt collector have a legal department within their company.