Collections call after a year - no prior notice

My wife gave birth to my son a little over a year ago on the 19th March 2014, <br> after settling all of the bills presented within two months, by means of cash and card, <br> Lo and behold, I now get a call from the Lake Smit & Partners collections department <br> telling me that I am behind in a payment from that birth, this is the first correspondence that I have<br> received from them in over a year, and am asked to produce cash slip proof of payment in regards<br> to settling the account. <br> Firstly , I don't know whether they have been asleep until now, to suddenly pick up that I have a bill outstanding ? <br> Secondly, How is it at all reasonable to expect me to locate a cash slip after such a long period has passed ? <br> Please assist urgently.<br> <br> Winston S<br>