Bank holiday entitlement

Hey, I have a question regarding bank holiday entitlement for part time workers.

If a part time worker works 4 hours on Sunday, 8 hours on Monday, 8 hours on Tuesday - what would be there entitlement to pay on the bank holiday (falling on a Monday that they work)? 

Payroll is saying that they should receive 4 hours based on their last shift worked (Sunday) but citizens information states that, “If you work for your employer for at least 40 hours in the 5 weeks before the public holiday and it falls on a day you normally work, you get paid for the day even if you don’t work. If you have to work that day, you are entitled to an extra day's pay” which would state to me that the worker should receive 8 hours BH pay on top of the 8 hours they worked (essentially double time).

Can anyone clarify please?