Incosistency and unfair treatment by Discovery

I became a member Discovery MA in Aug '08 when I joined Ernst & Young as an employee. Upon resigning in May '12 l continued cover as a private member. I was told becos I was no longer attached to a company, I would pay monthly contributions in advance. This carried on from June'12 to Feb '15. In this period from'08 my family and I had never put in any claims at all becos we are quite a healthy family. I joined Unilever as a permanent employee on 1st March. I was required to transfer my membership from private member to Unilever. This I did at end of Feb and actually went in at Discovery H/O myself. Becos my monthly d/o was on 1st of every month, it was confirmed that I can stop my d/o as I would now be debited through my salary. On 1sst March, I stopped the d/o. Discovery now claims that all along they had been debiting me in arrears therefore still owe the Feb premium and now refuse to load me on as a Unilever employee becos I owe them. It's rather strange that Discovery agrees that private members pay in advance but somehow mine was left in arrears. What's so special about me? Now I am without medical cover becos they don't want to load me on and are ruining my reputation @Unilev