Reply to neil:
It definitely doesn't help if the clutch brake is soaked with oil. Rinse it off with some kerosine - get the brush in there and give it all a good clean - the friction and the back of the throwout where it bears. You can use some gas in a pinch if necessary - remove the plug from the bottom of the case to let the build-up out. If the pilot is good, it should stop easily with a clean brake. If it still doesn't, then check that there isn't some foreign material in there binding on the clutch plates. If that's good, then I would actually suspect the pilot bearing.
I had one D2 that the previous owner had over greased the pilot bearing and the grease was holding the clutch disc to the flywheel. When good and warm all was well but cold, the clutch brake was useless. Engaging and disengaging the clutch was problematic, specially in tight quarters. Kick the clutch out and stand on the brakes was the order of the day until everything warmed up.
I put a couple gallons of diesel in the clutch housing and proceeded to work the clutch lever, turn engine about a third of a turn, more working the clutch lever, another 1/3 turn and more on the clutch lever. After about an hour, I could hear the clutch disc finally get loose. Drained the diesel and that, as they say was that.