You're not too late — but don't sit on it. The account stays in your name and can keep billing until they update their records, so the sooner you get in touch, the less there is to unpick.
Give them the date you actually moved out, not the date you're calling.
No final meter reading? They may estimate one. Ask what figure they've used and challenge it if it looks high. If the new occupant has already given an opening reading, that can help settle it — ask your supplier to confirm the dates and readings they've used.