Incorrect billing and ignoring emails

I was chased to settle my Nashua Mobile account and set up a monthly debit order. I asked for the final amount and was advised of an amount in writing. I then made the agreement to pay an amount monthly. I am now following up for confirmation that this will be the last payment and I have now been told I owe more. The amount they are saying that I owe almost doubles the amount that I was first advised of. I refuse to pay more than what was put in writing to me. I have now been ignored on ALL my emails and receive no details. I am just told every time I phone that I owe more money. No one is taking responsibility that they put an amount in writing to me that I agreed upon. unhelpful, rude and take no responsibility of mistakes made on their side and they expect me to suffer and just pay more money. Well I refuse to do so as I am sticking to the amount they have advised. This is my right and will take action further if they do not stick to the agreed amount they put in writing to me. I can not believe that they think they can get away with this. just because they are legal they think they can bully us into submission. I will pay the amount agreed only. They can pay the balance