This is a bit of an unclear area - needs a test case! As an employment lawyer I would advise:
It depends on the right in your contract.
All employees have the right to have 28 days leave a year (statutory minimum).
So if:
Your contract says 28 days a year (including bank holidays) - you will accrue 28 days during 12 months of maternity leave (pro rate for short periods).
Your contract says 35 days leave per year - you will accrue 35 days during 12 months of materinty leave.
Your contract says 28 days leave plus bank/public holidays - you will accrue 28 days. The right in your contract is to take a bank holiday off NOT a day of annual leave.
Your contract says 25 days plus bank holidays - you will accrue 28 days because of the statutory minimum.
There may also be provisions on carry over of holiday from one year to the next. Currently the Working Time Regs say that holiday cannot be carried over (so if your leave spans a holiday year you may lose any accrued holiday). The contractual holiday (over the 28 days will depend on your contract.) There is a ECJ case out there at the moment which suggests this may not be the case but it is not clear under UK law. Equally you may have arguments if your emplyoer allows other people to take a day in lieu of a bank holiday.
Public sector contracts (and employment policies) tend to be more generous on this type of entitlement. If you post the contractual wording I'll let you have my view on whether to take advice but the advie your HR received is largely correct if confusingly phrased.
If you are looking for places to back you up on the internet HR will be able to find comparable sources which state the opposite.