Vodafone Have overcharged me at least over £10 for the last 22 months Internet

 Since I have taken out a new contact with Vodafone in January 2016 they have been overcharging me every monthly bill since then. I was originally supposed to have a 20% discount as my auntie works in the NHS, but Vodafone didn't add on my discount and then conveniently told me it was too late to add on the discount after I had purposely taken out a more expensive contract than I can afford with them. In my previous contract I paid £15 a month which suited me great as I am a university student, I now pay £47 a month. I normally get overcharged to around £60 every month and have to repeatedly call up/email/visit store to get a refund. I have gone into my overdraft numerous times because of this and not once have they offered me any apologies or discount.