When should my final bill be from.

My 18 month broadband contract had come to an end with plusnet so on the 9th of august they sent me an email to say they will be taking a payment of £29.98 and this will leave my account on or after 14/08/2018.
I then thought this was to expensive so I tried to get a better deal but they couldn't match other suppliers so I signed up to nowtv on the 13th of august and the activation date was the 29th of august. The 29th of august came and my plusnet service was cut off about dinner time but now tv didn't get me activated until the 3rd of September due to a problem with me connected to someone else address.
On the 11/9/2018 I received an email from plusnet saying we're sad to hear that you've requested to cancel your Plusnet services, but we've done as you asked and placed your cancellation request. The cancellation will take place on 11th September 2018 as soon as we've worked out the final fees.
Now yesterday I received another email from plusnet and they are trying to charged me £14.49 for broadband only from the 29th of august and they will be taking the money from my account on or after 19th September 2018.