I would say that the majority of employers wouldnt allow bank holiday hours ontop of your maternity unless bank holidays are included in your annual leave allowance
found this on the working famillies site
Annual shutdowns and bank holidays
The Working Time Regulations do not say what happens if you are on maternity leave during a
fixed holiday such as an annual shutdown or bank holiday. At present such holidays are normally
lost and cannot be taken at another time. However, in a recent case called Merino Gomez v
Continental Industrias Del Caucho the European Court of Justice said that if a womans paid
statutory annual leave coincides with her maternity leave, she must be allowed to take her annual
leave at another time, if not this would deny her the right to paid annual leave. The Court said that
the reasons for annual leave and maternity leave are different and that a worker is entitled to the
benefit of both.
If there is a bank holiday during your maternity leave you are not normally entitled to take a day
off at another time. If your workplace allows employees who are sick on a bank holiday to take a
day off in lieu (this is unusual) you could argue that it would be sex discrimination to refuse to
allow a woman on maternity leave to do the same. If you are entitled to 20/24 days holiday
including bank holidays you could also argue that, following the Gomez case, you should be able
to take bank holidays at another time. However, there have not yet been any tribunal cases to test