EE 12 month discount expiry on 18 month contract

Emmpii

Hi,

I was on EE a year ago and got myself an "upgrade". Over the online chat, the agent approved my transfer to a new SIMO 20gb plan for £16pm. That's all I was told. I assumed it would be a 12 month contract as usual but turned out it was 18 months. Few days later I asked why that was case and I was told it's because this was a 'customised' plan.

Anyway, so I reached the end of 12 months and this month I've been billed £20. I asked the online support and I was told it's because in the manager notes it says I was only to be discounted for 12 months and for the other 6 months I'd pay £20. Looking at the transcript from when I signed up, none of that was said to me.
In fact the online agent even said this:

EE_Agent wrote:

it doesn't need to be in your emails/chats/letters, discount is not part of your contract it is discretionary so if there are notes from a manager stating it was for 12 months then im afraid you are not entitled to have it for any longer than that

What options do I have here? Essentially upgrading to any plan could mean I'm only being given a 'hidden discount' which expires before my actual contract does.