If you have the Mini-Pure System sold pre-assembled at GBH then you have 2 filter housings upstream of the UV light and one filter housing downstream of the light. The standard recommendation for UV Systems is for the UV light to be the final component with no filters downstream. The reason for this is that when you lose power you can still flow water through your system without exposing it to the UV light due to the residual pressure in your bladder tank air bag and potentially contaminate your downstream filter. Typically the last filter is a charcoal filter and charcoal provides an excellent breeding ground for bacteria.
The primary things you are filtering from cistern water is Saharan Dust and other very fine particulate matter that washes off your roof, so you don't really need two particulate filters anyway. A two sediment filter system is typically for water with various sizes of particulate matter, such as a lake or river, so the upstream filter removes the larger particulate that would quickly plug up the finer downstream filter. The charcoal filter enhances taste and odor as well as acting as a sediment filter, but typically plugs up quicker than a standard sediment filter.
I would recommend installing a 5 or 10 micron sediment filter (string or poly) in the upstream filter housing, a 10 micron solid charcoal block filter in the second filter housing, and leave the downstream filter housing empty. The pleated charcoal filters you might see in the store are worthless.
Tropical Plumbing usually has a fair selection of filters but your cheapest option is to order a case of each filter type on-line and have them shipped here. With that size filter (4.5" by 20") you probably need to initially change them after six months and then base future replacements on their appearance. If very dirty then maybe every 4 months, if fairly clean then maybe 9 months and check again. It depends on the amount of water that flows through the filter and how dirty that water is.