Resigning from my job – Paying back holiday?

I'm in a bit of a predicament and just wanted to get some insight before speaking to my line-manager.

I work part time (18.5 hrs a week) and I will be handing in my resignation.

My annual leave is taken in hours and not days. My annual entitlement is 118.4hrs, I have taken 89.5hrs and I have 28.90hrs remaining.

The contract states: If an employee joins or leaves part way through a year the leave entitlement is calculated for complete calendar months only and rounded to the nearest half day.

How much time would I have to owe back to my employer?

Thanks