Yesterday i received payment from them after about 10 days for three seperate invoices and had numerous this year.
Two other forum members i know have also got a refund within a short period of time.
All you have to do is POST them the original envelope and invoice (i keep photocopies) showing the duty paid (NOT email them) accompanied with a brief letter or the form you can print off, asking for your refund and with your bank details.
If your buying a few its also easier to send them together, maybe at the end of each month.