Sorry I missed your reply. I do appreciate your understanding.
My online bill dated 31 March for £62.49, as well as having my £10 credit, says that £10 was owed from the previous bill, which is nonsense by the way, and of course that £10 debit then cancelled out my £10 credit!
You will notice that all my online bills have been for about £62, and I have never seen one with the £10 credit added to make £52 yet, nor have I owed £10, as claimed.
I would be grateful if you would investigate this further.