You continue to accrue holidays as normal whilst on mat leave.
Does your holiday year run from April to March? Did you take all of your full year's entitlement in April?
When you leave, you are entitled to holidays at a pro rata amount to reflect how much of the holiday year you were employed.
So say you left at the end of Feb and your holiday year runs from April to March, you are entitled to 11/12ths of your full year entitlement. (e.g if your full year entitlement is 30 days, this would be 27.5 days). They would then take off anything you have already used and pay you for anything you haven't taken. (e.g. if you've used 20 days, you'd be paid for 7.5 days)
So, if you used your full year entitlement in April, then potentially you will have overused your annual leave. (e.g. if you used 30 days, you would have over-used by 2.5 days). This would then be deducted from your final salary.