It appears this is not an isolated case.
Having cancelled my direct debit I was contacted and offered 3 months credit of £109.47.
I declined this and told them I still wished to cancel. I was informed fine my final bill would be ready in 7-10 days.
My March bill arrived dated 25th and was for £36.49 this was prior to the 3 months credit offer.
My BT Sport access ended 1st April 2020
Final bill has arrived today and lo and behold it is for £123.60.
I have had £22.36 refunded for my BT Sport but have been charged for the 3 months credit, described as money refunded to you.
So everyone please be vigilant check any bills as BT will appear to try anything to rip us off.