Required - Easy maintenance filter!

My parents currently run a Cascade 1500 pump, connected to one of the
old Cyprio Planter 500 filters. This filter is some age now, and the
UVC sheath is apparently sealed in and difficult to remove (and also
very dirty on the surface).

The pond is about 470 UK gallons, and the above pump has a max flowrate
of 1500 litres (330 GPH). Now I understand that the 500gallon filter
is supposedly max 250GPH. The filter is becoming a bind to clean and
I'm dubious about its efficiency actually. There is probably about
4-5ft of fish in there. Recommendations please? If the pump is more
than 250GPH, should I be looking at filter that take a flowrate higher
than this? or is a 250GPH filter still ok? Mt feeling is that I
should look at Ecomax 4500 (500GPH max).

Suggestions/comments welcome. They do not want to buy another pump.


--
psbderbys