Being a SPI it is incredible difficult for one cylinder (or in this case two) to over fuel where the others don't this points to a problem with these two cylinders. given the circumstances of your complaint i would suspect that if you were to do a slow compression test on these two cylinders you would find that they drop pressure over time much quicker than the other two even if on a quick test the compression all looks fine.
basically these cylinders are lessing exhaust gasses bypass the the piston rings and oil scrapers, which builds pressure in the crank case but also vaporizes the oil through excessive heat hence why the oil is then able to travel up the breather pipe and into the intake system, you always get a small amount of oil vapour through the breather pipe thats what its there for but you describe a strong smell of oil when running with the pipe disconnected which suggests a far greater amount of oil is being lost this way.
secondly the lost of seal allows unburn fuel into the crank case which replaces the oil and thus the level doesn't change very much. a favorite on some landrovers when this happens is the oil gets replaced with fuel which is of course thinner than oil leading to the engine seizing.
if there is lost compression in these cylinders it will cause problems with the fuel air mix which can lead to incomplete combustion and in this case blackened plugs.
there are a lot of things that has lead Fingers to his original suggestion not just a random guess at a problem and in this case you need to take his advice and get a proper compression test done wet and dry to check what is really going on, do the compression slowly and see if it drops off over a short space of time etc.
as for water loss there are a number of things that can lead to water being burnt off. sticking thermostat leaky gaskets poor seal on the water bottle cap leading to pressure in the system being lost. or you could indeed have the beginnings of a knackered head gasket (its not a new car after all) in which case i would be expecting to see a lot of steam/smoke from the exhaust, but this time of year you always get a bit of smoke/steam in the mornings anyway.
before looking any further into possible causes first rule out the obvious with a compression test, which you can buy the kit from any halfords tonight for about £20? and get your results up here in a couple of hrs
