Whatever you do - they will probably rumble you eventually and try to recover the arrears. You do not have a leg to stand on if it was ever to become a legal battle.
If it were me - I would contact them and complain that they have not been charging me. I would argue that it is their fault and that you cannot afford to pay the arrears in one lump. You may find that they accept your argument and simoly start to charge as from now - alternatively you may be able to get them to spread the arrears over future payments.
I feel that the longer you leave it - the worse the outcome will be.