[Mr Angry]

As Kymmy says, you are not "getting it".

Wescot, based on their business model, have in all probability "future purchased" the debt as part of a bulk debt purchase from O2 who had already written it off.

Up until you made your payment by card you / Wescot had no legal relationship nor was there any debt acknowledged.

This is why I advised you to ask them to provide you with a copy of the deed of assignment from O2 before proceeding beyond acknowledging their letter. If they do not have one, which you can be 99.9% sure they don't, then they have no legal right to purport to collect a debt of behalf of O2.

However, this is all academic now that you've made a payment.

Unfortunately things will unfold pretty much as Kymmy has described.