Ratter,
Having tried to follow everything said, and getting rather confused, I've attached the link to the ACAS Holiday and Holiday Pay document, which is what I used to refer to when discussing this matter with my part time staff, although we didn't work weekends, but this doesn't really make any difference to the overall concept:
http://www.acas.org.uk/media/pdf/d/4/Holidays-and-holiday-pay.pdf
Assuming that your contract states that you work 4 days during the 7 day working week, your minimum amount of holidays due over the course of a year is 4 times 5.6 days, which equals 22.4 days.
Working overtime , either at the end of your shift, or on your normal days off, makes no difference to your holiday entitlement.
If your contract states that you are entitled to 28 days holiday over the course of the holiday year, then your employers are being generous, as this is the minimum entitlement of a full time worker (they may have made a mistake and issued you with a contract for a full timer in error, not realising the difference!), or have you earned extra holiday due to length of service?
Whether paid Bank Holidays are included within your holiday entitlement should be covered within your contract, especially working within an area which cannot shut down for a day. Having worked within the Financial Services sector for all my working life, obviously we would shut down on Bank Holidays as far a customers were concerned, although I would often have to work due to systems testing and implementation, but we would be paid at an appropriate rate - this was before the Working Time Directive and minimum holidays!
I would argue that if you take a week off on holiday, this represents 4 working days only, as per contract and that you were entitled to a minimum of 5.6 such periods, or 7 should your contract actually stipulate 28 days annual leave.