Switched from plus.net to sky and getting charged from both.

Lloydmcallister

Hey guys was looking for some advice, so basically in November 2018 I decided to switch my broadband provider from plusnet to sky as plusnets customer service was awful and was having connectivity issues. Fast forward to March 2019 and I realised I was still being charged from plusnet. My fault I wasn’t paying close enough attention, sky admitted it was there responsibility to make plusnet aware that I had switched and that the best they could do was offer me half line rental for 3 months(better than nothing I guess).
Then last week I received a letter from a debt collection agency stating I owed plus.net £20 and a additional fee of £5. I called plus.net and the customer rep was really helpful and very apologetic assured me that it was a mistake and had it cleared. I then received a second letter stating I still owe the money. I called plusnet today and they basically told me it’s out of there hands and is nothing to do with them. Called sky and the arrogant rep told me that I had already received a discount on my bill and that’s all I’m going to get.
What I’m wondering here is should I pay this bill? I’ve never had any debt and always pay every bill on time so I want to keep my perfect credit score.
Thanks in advance for any advice.