Hi, I was sent a filter by BT which it turns out I don't need. Do I have to pay to return it?
Not if its just a microfilter, as it would cost more to return it, that its worth.
See link about returning BT equipment.
Note it is dependent on when you received your equipment whether or not you have to return it to BT.
I am aware that it does not apply to filters but the bit about not going to the Post Office to return any equipment does apply.