Grr just lost all my post by accidentally sweeping back.
Ok that's a decent sized koi pond. I have a feeling that you're closer to 20K..
They are using CH circulating pumps in 1 and by the looks of it the system has an inbuilt reversal system. Each of the pumps have a sluice gate valve, the one open and the pump at the moment is normal operation. That pumps out through the UV.
If I'm right the tubes in 8 are designed to output into the drain on the reverse but also act as an overflow protection - so if it rains a lot you don't have to manually take water out of the pond.
Oddly I replaced my CH pump recently.. so I have some experience. The number selector sets the pump rate - yours is set at 1, or according to the label 2400l/min. As the pump is well within it's normal pump head height (no centre base drain in the pond I take it) that figure of 2400l/min is about right. So if that's correct you're running at 144,400 l/hour max or say 80-100K to take in any form of head or flowrate blocks (i.e. filter). This is why I think your pond is probably bigger than you think possibly 20K as you want to filter all water at least once an hour (goldfish are happy with once every 2-3 hours). A heavily stocked pond needs more filtration (more blocking load).
Btw - the CH pump ISN'T water tight on the electrics, so be careful (at least the Grundfos pump I fitted wasn't when you connect up the electrics). There's an arrow on the back of the pipe housing - i.e. the bit where the pipes connect in showing the flow directions, you can't use the orientation of the main housing as an indication of flow direction because you can unscrew & rotate to fit depending on the CH installation).
To clean the normal pump is switched off, the valve closed and the INPUT valve is closed too (there should be something near the input - an I assume there's a pump there too?), the other pump (pumping backwards) is connected - possibly with a removable flexible hose into the pond (or even a pond vacuum), and the valve opened. Pumping backwards agitate the filters and the now very dirty crappy water then overspills down the drain in 8.
The reason I suggest vacuum is that without a bottom drain (3 is an overflow skimmer that takes water but don't really clean the crap out the bottom of the pond), you'll need to get the poo from the 6ft depths in the centre. It maybe there is a centre drain that connects to the 6 cyclone but is blocked some how. Either way part of the normal maintenance without something to suck the pop from the bottom would need to be a vacuum of some description.
If I'm right then 6 is a cyclone bin used to remove the big waste like a dyson vac. The way it cleans is that the reverse water (and a bit of poking) over flows the wall and into 7 or 8. Unless there's a valve system on this too to switch dirty flow.
You can check if 6 is a cyclone by looking to see if there's a slit where a pump would push water from 3 around into a cyclone. Easy to spot - just run a cane around the inside and see if there's a slit. It should be about 1/3-1/4 of the way down. Cyclones need a pump to drive the vertex (unless it's sealed - which this isn't). The pump would normally pull the real heavy waste from the centre of the depth, though a CH pump then through the slit.. this causes the vertex, the solid settles out, and the water then exits the top (a waste vent at the bottom would be closed with a valve) the top cleaner water then moves through into the next stage of the filtration system - i.e. the first bin.
Are you sure that the sump pump is not the input pump into 6 that's disconnected? If not it could be a design like this:
http://www.thepondforum.com/showthread.php/72-DIY-55-Gallon-Barrel-Pond-Filter
UV - you'll need to replace this bulb every 6-12 months, clean the quartz tube - be VERY careful of UV and ensure that the thing is off before you open it.
Seems that's a very advanced filter setup, but needs lots of work to get it back running - but the heavy work is already done.. a 10Kl/h ezpod is about £500-600.. so you'll probably be looking at 1500-2000 if you chose to rip it out and start again (including air pump).
The estate agent has an air pump sat on the wall.. that's something you'll want for koi - or at least a load of white water to maximise O2/CO2.
Evaporation is probably quite high on that surface area too.. so check if they don't have a large rainwater butt around. If the skimmer (3) is the only input into filtering, if the water level drops too much dir to evaporation then the filtering gets shut off..
As the previous owner if there's a base drain in the centre of the pond connected to 6.. it could save you a shed load of time in maintenance..
I joined extremekoi as they're UK based.. but like most thing you'll gravitate as you start searching. I'm assuming the others here will chime in with suggestions 