As long as your
Subject access request
was expressed in the way which would be seen to refer to the two accounts, then they are in breach of their statutory obligations even though they have returned the £10 to you.
If you have got some evidence of charges on the data that they have given to you, then I would suggest that you put a claim in for that immediately and at the same time claim for the data protection non-compliance. Once they receive the claim, they will get themselves into gear and they will provide you with the rest of the data. You can then go on to amend the claim to include the further charges which the new data may reveal.
In your claim make it clear that you reserve the right to amend and add further charges once you receive the data which is missing from the original disclosure