Defender 2.4 Clutch slipping

I currently have an issue that I was hoping someone might be able to shed some light on.

My 2.4 110 has just starting slipping in 6th gear. It impossible to get the clutch to slip in any other gear even under extreme load. The strange thing is the clutch is only a few months old. I also had the motor replaced a couple of months ago and my mechanic said the clutch looked like new. I bled the system yesterday and the fluid was very black and there was also a large amount of black, sticky residue all around the top of the clutch reservoir. The slave cylinder is new as that also failed on me less than 6 months ago (haven't been having a good run). It's going to the mechanic on Tuesday but any thoughts/suggestions would be appreciated. Cheers.

Black brake fluid is an indication of seals breaking down in either the master or slave cylinder, as the slave cylinder has already been changed it would most likely be the master cylinder unless the fluid was left contaminated and was not changed from when the slave was changed, But this would not explain why the clutch is slipping as once gears are engaged it should have no effect on the drive system unless there is a leak of fluid or oil on to the clutch plate, is there any fluid level drop. (I understand on the 2.4 ford engine that the slave cylinder is inside the bell housing as part of the release bearing) warning: never operate the new slave before fitting and bleeding correctly as it will bugger them up. I know someone who did and had to replace it again.
Are there any other issues like binding brakes that could be causing this. ?

Generally speaking a slipping clutch is either, contaminated friction plate due to oil etc (often from rear crank seal), worn clutch plate, clutch hydraulics not fully disengaging. I don't know much about this particular engine but I'd be checking the bell housing drain plug to see if oil is leaking out, checking the clutch adjuster to see if it has enough free play.