Lindy Lu:
Firstly, thank you for my 15,000 points.
Possibly you have already found this out but the current TC minimum payment is 2.5%. At a typical 19.9% annual rate it's impossible to go over limit while maintaining minimum payments if the card is not being used.
Some payment glitch must have occurred somewhere. If not your payment then it's down to TC.
Financial complaints often take an age but remember if you are not happy 8 weeks after first making the complaint, you are then entitled to refer the complaint to the Financial Ombudsman System. If you get to 7 weeks I would tell the company what outcome you want within 7 days, and make it clear you will invoke the FOS. That will cost the company a hefty case fee, you nothing. The threat ought to be sufficient.