Assumptions :
All accrued holiday was taken as part of maternity absence - so 27th May is effectively day zero for holiday calculations.
8 Bank Holidays in the Year
20 Days Holiday for a Full Time worker
25 hours is worked as 5 x 5 hour days
She will work for 31 weeks of the 52 this holiday year - thus she is entitled to (31/52) multiplied by (20+8) days holiday. Which is 16.7 days (of five hours per day).
She will need to use 4 of those for Bank Holidays (May 27th, Aug 26th, Christmas Day, Boxing Day). So she will have 12.7 days to take as other holiday - about two and a half weeks).
Dave