Just to get it off my chest.... mixed up some SLC today, some from a new pack, some from an old. D'oh. After mixing, it was pretty obvious that the stuff wasn't going to self-level - it went on more like plaster. Ho hum, I think, might as well chuck it down anyway. D'oh number 2. Do a pretty good job of getting it smooth and reasonably level, at least on the thick bits, which makes it worth it.
Wait four hours, and notice that it's a) sunk a bit in places, and b) not really setting that well.
Scrape it all off, sweep up, and have cup of tea. I must not use old powder. I must not use old powder.