So I'm running an external oil catch can with breather filter, while topping up rad fluid today I noticed it is covered and blocked up with what looks like oily sludge. I took out the oil cap and the same stuff is on the inside of the cap. What's going on?? It's probably due for an oil change but is there anyway of flushing out whatever the gunk is, obviously it's in the motor too..

If there is a light amount of sludge under the oil cap, it could be due to water vapour being trapped from high humidity [Sth East QLD}, but if it is also present in the radiator, I think a blown head gasket is more likely to be the cause. Dont discount that it may be possible that water is entering the engine via the front timimg cover. The covers have been known to corrode [around the water pump area] if the recommended anti freeze/boil/corrode fluids have not been used.

Do some testing like cylinder compression, pressurise the water system and observe if there is a leak down.

If you have mayonaise like paste on the underside of your filler cap but not on your dipstick, then it's condensation from blowby. (H2O is a byproduct of combustion) and since you have a catch can which is a passive crankcase ventilation system, it will accumulate. Whereas the factory positve crankcase ventilation will draw away this condensation and reburn it.
But if you have that goop on your dipstick too, then you've got a breach in your cooling system. (Most likely a blown head gasket, or cracked head)

is it a major issue if its just from blowby? i understand heaps of build up in the rocker cover could be an issue but would it not just evaporate again when the car gets up to temp and then out the catch can breather it goes?