HSBC Credit Card
Charging Late payment fees without any valid reason

I'm using a HSBC Credit Card and the card no. is 4476929996743354. In my Aug statement dated Aug 16, the HSBC bank has charged INR 750/- as the late payment fee. As there was a bank strike going on on Aug 6th & 7th and all their ATMs were closed, I couldn't drop the cheque on 6th. The bank people called me in the morning of 7th for the payment enquiry and I asked them to send their person to my office and on the same day I gave them two cheques of INR - & 415/-. Now their argument is that, the cheque should be dropped at least 3 days prior to the due date. But my point is that, I've always put the cheque on the due date, and no money was deducted previously due to this reason. Then what happened to this case?

Please look into the matter.