Annual Passholder buying extra tickets

[BPTP]

We're going at the end of August and taking along Gran & nephews. We have annual passes but we need to buy passes for the. We arrive quite late but it's the last night of fantullusion & fireworks & so we want to go straight to the park for that night. I think realistically we won't get to the park until 7pm. I know you can get a cheaper 'evening pass' but I think it was just as cheap to buy five day passes (I did see prices somewhere, but can't remember them at the moment!). Anyway, can we just buy five day passes at the gate in the evening & show our passes to get the discount or should we try and buy them in advance?