First off try bleeding the slave cylinder properly.If you can get under the car drill a hole so that it faces the plate somewhere near the edge.Place a small pipe(bit bigger than a WD40 pipe) push it just through the hole and give the clutch a good squirt of de-greaser and try car.If it still slipping and if all else fails a good dose of Coca-Cola will clean off crap better than anything else.
this happened to a freind with a e 39 540 too.the flywheel either hadnt been machined or hadnt been machined properly or the dual mass cant be machined....
Gone but not forgotten
In rare cases, the slave cylinder push rod is not fully seated into the cup of the release lever ( or the cup of the slave itself, depending on the design): were that the case, there would always be excessive pressure applied to the pressure plate ...... taking only a small movement of the pedal to release the clutch.