Aol/talk talk

To cut a long story short, I had been a customer of AOL which I have since found out is TALK TALK where I had a direct debit in place for £24.99 month. In March this year one of their sales reps contacted me to offer me a reduction in my repayments to £19.99 if I commit to them for another 12 months to which I agreed and as a result they would send me an email confirmation which I had 24 or 48hrs to consider and withdraw if I chose. NO SUCH EMAIL ARRRIVED. This is where the fun began!!!!!!!! The following month my payments more than doubled ie £47 ish per month hence a phone call to sort out their incompetence at MY COST. Anyway, several months later, still no refund and monies being taken from my bank account went down once but then straight back up again to more than double the agreement. Despite several phone calls where they blamed their computers and not human error input into computers and despite several emails to AOL customer service I got nowhere so I cancelled my direct debit. In September, they removed my access to the internet requesting monies from me to reinstate despite the fact I still had not received any refund of already overpaid monies which resulted in yet another phone call where they agreed they owed ME £80 and where I demanded to withdraw my commitment due to their incompetence which they did and also apologising for all the inconvenience they had caused me and that this would be the end of the matter and I would get my refund. To my horror, on Monday of this week I received a call from Advantis Debt Collecting Agency requesting nearly £500 in respect of money AOL/TALK TALK say I owe them. Having read your adivce about a BREACH in verbal contract where they have (Where a term is fundamental to the contract and a breach of that term has had significant consequences for you, you have the right to terminate the contract) am I in a position to fight this because I have never been in debt with anyone and this is causing me sleepless nights together with anxiety and stress.