Realised I had been paying full contract price when contract ran out months prior.
Thought I had changed this successfully after receiving email confirming my new billing price to be £14 for same data, calls and text allowance, yet now it’s two months later I receive another email claiming my usual DD of £40+ will be taken soon. 
I understood the final payment/last months charge would be the higher price yet do not understand why my billing info page says I will only pay £5 this month, yet receiving emails that ID will charge old contract price.