Help with bill overcharged or misunderstanding

Hello

I'm having bill issue. I renegotiate my contract to £40.01 a month on 1gb with no activision fees and made sure no one off fees at all on the call. However I don't understand the bills it all over the place to me and even using the site breakdown I don't get what I'm missing. My bill has not been £40.01 a month yet. It's been from £39 to £66. I understand this could be me misunderstanding something but i've never missed a payment for years and now I'm getting calls and texts saying I need to pay £18odds due to late payments. I will also point out I have never had 1gb speeds since the switch. Most nights its all over the show. Like 2 nights it was at 150 on speed tests but for now lets deal with the bill and I will call one night when its bad.

I just paid £40.50 on due datish and then I got texts and calls to pay £18 because im late so I paid £19. Can someone help me understand so I can get my bill to start at the contact £40.01 a month.

Thank you for your time.