Bought unlimited class pack, but classes have been made chargeable

I bought an unlimited classes cult fit pack. It is advised as having the ability to book unlimited no of classes across any cult centre for the duration of the pack. I bought this pack only after they introduced swimming as an option, I had contacted them earlier as well to enquire about when they would add swimming and bought the pack only when it was added as a class. I bought the 6 month unlimited pack which was offered with a 2 month extension as part of one of their always ongoing sales. Last week I got a mail informing that they would start charging for swimming classes, which goes against what they promised me when selling their 'unlimited' pack, for which I shelled out close to Rs. protected/-. I therefore raised a query, and they reverted that they'd be charging protected/class since the economics isn't working out for them. Hence, I asked them for a refund since it was not the product I bought and now works out to be much costlier than earlier advertised. Their response to this was that they would provide a pro rated refund, which was fine by me since I would obviously pay for services used. But their condition is they would calculate the pro rata only on basis of the 6 months and would not consider the 2 month extension. My contention is that 1. I bought the pack keeping in mind the cost over a period of 8 months and hence calculated the cost and made the purchase. 2. Early termination of the pack is because of their inability to live up to their promise of unlimited classes by adding a charge per class and not due to any fault of mine so I should get a refund pro rated on the original 8 month period. I wish that my consumer complaint will get resolved soon.