Agree that you need to find out how your company do it.
The companies that I have worked for usually work out your TOTAL holiday entitlement pro rata.
So, a full timer would get 25 days plus 8 bank holidays every year (last year everyone got an extra day for the Royal Wedding but there are 8 bank holidays as standard).
So, a full timer would get 33 days. If you do a 3 day week, you get three fifths of that so 19.8 days which would be rounded up as 20 days.
Then, if a bank holiday falls on one of your normal work days (i.e. a MOnday, TUesday or a Wednesday) you have to take it as annual leave. At my firm, this is done automatically for part timers so they know where they are upto.
As a complete aside, do you have to work MOndays? I have worked 3 days for 8 years, and obviously I get the same amount of leave (20 days) whichever 3 days I work. However, I have never worked Mondays - even ignoring Christmas and New Year, there are ALWAYS 4 bank holidays on a Monday, so by working Mondays, you've used up 4 days of your leave straight away. If Christmas Day is a MOnday, Boxing Day would be a Tuesday, and New Years Day would also be a MOnday, that means that you'd have to use 7 days just to cover the bank holidays (the only bank holiday you wouldn't need to take annual leave for would be Good Friday). If you think you only get 3/5 of the 8 days added into your leave, so 4.8 days (probably round up to 5) you'll spend more of your own holiday leave covering Bank Hols (if that makes sense) by working MOndays.