Default on credit report

[Sam11a]

I have recently applied for a mortgage and have just discovered a default on my credit file. It related to a Sky agreement. We cancelled the service as our new property could not have a dish. Everything was paid. We returned the equipment. This is in 2021. 
I spoke with Sky today about the default. They say I have nothing to pay to them as they have sold this debt to an outside agency. 
My argument with Sky was never concluded. They agreed that I had returned the equipment  - this is what the £115 debt related to. But they claim I didn't return it in time. I was therefore charged for the late return. What I wanted to try and clarify was their proof of return and their proof that it was late, I don't believe it was. They could not supply this. Reading through all the forms here, it seems like, it is a pretty regular occurrence. Three years later, I don't have any proof of postage but I do have agreement from Sky that the equipment was returned. I don't feel like the debt should have been added to my account in the first place. Therefore it should never have been sold on. And if Sky can't prove to me that the return was late, then where do I stand on getting the default removed from my credit file. Their information was wrong about how I paid for the account. 
Any advice in sorting this mess out?

[GD1]

@Sam11a  Without proof you returned it within the specified time it's your word against Skys, therefore you may need to raise dispute with both the collection agent & your CRA.