No debt owed!

I recently changed banking details. Sequence of events, submitted written request by my new bank well in advance of debit order date, telephone call from Netstar confirming changed, debit order doesn't go off on agreed date from neither account, 1st working day after I contact Netstar in writing and telephonically, apology received and again confirmed bank account details and receive confirmation in writing from Netstar all is sorted, that same afternoon I get an sms from Hybrid Collections threatening legal action for the very same payment that was 'solved', contact Netstar again in writing get a response I'm not their client, phone again and again confirmed all in order, receive written confirmation again that problem is resolved and low and behold, 2 days later a call from Hybrid Collections threatening legal action and when the lady is informed that there is no need for her as the problem has been resolved, I get told people like me are the worst to do business with. Hybrid Collections has to learn about professionalism.