Wow. Looks quite bad, actually. That shit's going to be right through your engine block too. Thankfully the only moving parts it will impact are the water pump and thermostat, but it could block smaller galleyways - I don't know if the D22 turbo is water cooled like the D40, but it's something to be concerned about, along with the major one - the radiator cores themselves. A flow test is needed.
As to where it came from, who knows? Someone poured some shit in there at some point just to piss you off? Were you 4x4ing, clogged the front up with mud after getting a hole, it sucked the shit in? Metal corrosion from inside somewhere?
Where did you fill the radiator from, a dirty water tank?
You should be using coolant mate. Did it still run OK?
As said above, you need to completely clean radiator, water pump and give the engine a thorough power flush.
Possibly someone at some stage having used 2 different coolants? They do react with each other, but I can't say I've seen what happens when they do, just a thought...
As above though, full flush of everything. You'll have to take the radiator to a radiator shop and have the core flushed (the remove the end tanks and flush it properly), unless you just want to buy a new one to replace it....
There shouldn't be any water In there as it was premix that went in there last time I flushed the radiator.
I checked the oil to make sure it wasn't milky and don't think it was oil on the radiator.
The stuff seems more of a sludge then dirt and could just be rust that has made its way to the top.
Will have to do another flush and get the radiator cleaned out aswell. Or possibly buy a new one.
Speaking of a new radiator, do these plastic tank radiators have a useby date? My D22 is 15 years old now and I have wondered if they be become brittle over time. I would prefer to not be stranded with a rooted radiator. I assume there is an all metal aftermarket that I can install? It still looks ok, but... plastic...