On the assumption that your previous accountant had made it clear to you at the outset what his charging structure would be, and you agreed to his terms, then yes, your accountant is perfectly entitled to retain your records while his fees are unpaid (technically, the right to retain records is called a lien).
You would have to show that he was negligent or dishonest in his dealings with you to avoid that result.
You can request his professional institute to get involved if you feel you have been overcharged, and they may review the case. You can also ask the Institute to pressure him to return your records.
Your new accountant can also write to the old ones advising them that the retention of records may prevent the completion and submission of your returns for the deadline. They could also mention that this may result in penalties or interest being incurred as a result of their refusal and they could be held responsible for meeting these.