ok, this is what happened..
I called up on the 5th of march to cancel my cashback contract (400min anytime X-net + 200 txts) which was meant to expire on the 7th of april.
Their sales team offered me another upgrade deal £8.50 p/m (£100 credit).
I accepted the deal. They told me the upgrade will begin at the end of my originally contract on the next billing month.
I called up today to check how many minutes I had left and was shocked to discover they've changed mu tariff!!! They've changed my tariff one month early:x
I called up to complain but their costumer sales team was useless and didn't see what the problem was. now I'm waiting for the manager to call me back :x