Thanks for your replies!
Fabrice - it's cool you managed to get a flower, well the aborted stalks are a problem also with my U.amethystina, it's very frustrating because each time it sends up flowerstalks I hope to see the flowers open - then nothing!
Ron - looking forward to hear how's the plant doing on the windowsill. As for the large pots, I use one of a larger size, pure LFS as substrate - the plant grows like crazy, lots of leaves, but not even a chance for a flower. I think I'll try to flood it.
Other toughts?