Pond/Filter system

Planning is nearly done & what it looks like I'll be building are two ponds.
One will be a raised rectangualr pond - about 800-850 UK Gallons (just over
1000 USG) and the second will be a half raised Liner pond which will be
about the same size. The Raised pond will be about 3'6" deep & will be for
Koi & the Liner pond will be 2-3 ft deep and for Goldfish, Shubunkins (Sp?)
etc. The two ponds will be connected by a waterfall which will flow out of
the raised pond and down into the liner pond. The difference in the water
levels will be about 2 foot. The two ponds are going into an area with a
very slight slope with the raised pond being slightly higher up the hill if
that makes sense.

The raised pond will be built from Concrete block, rendered and painted
inside and the edges will be capped with blue engineering bricks to match
the terrace walls.

The big dilemma now is about the filters.

My understanding is that it would be better to have a filter that's gravity
fed from the lower pond i.e. there is a side drain or a bottom drain that
links to a filter box in which the water level is basically at the same
height as the water in the pond. This then overflows into some kind of pump
chamber so that water is then pumped back into the pond. The advantage I can
see of this is that the pump is kept clean because its on the clean side of
the filter and the waste is not liquidized by a pump before it goes into the
filter

All well & good in theory but the problem I can see is in a couple of areas.
Firstly the pump chamber would need to be quite large and secondly the
situation as to what happens in the event that there is a leak or a
significant loss of water through evaporation - the whole thing could just
stop working. It would seem to be very sensitive to the water level in the
pond(s).

So the question is as to whether this kind of system is worth pursuing & if
so how do you rig it to cater for variations in water level or do I just
chuck a submersible pump in the lower pond & pump upto a filter box at the
top end of the system?

rgds

I.