Billing issues

On 1 March, I received the annual tariff increase email. In this mail, there is an offer to get 1 month's free subscription and to pay at the old tariff, if i pay my full year's subscription by 31 March. I was foolish enough to do this, but have been battling since then - First I was under-billed, then over-billed and now they are not processing my debit order. Should I pay cash and then battle to get my money back if the annual subscription is debited directly later? OR Should I wait until my service is stopped due to non-payment if the debit is not processed? Please help. to resolve my billing issue.