Bathroom Sink back fills / very slowly drains

[rugrat]

Hi
My bathroom sink backfills and very slowly drains away. I have removed the u- bend pipes, cleaned them, and put them back. When disconnected i have filled the wast pipe to see if this is blocked, it is fine.

When i put the u-bend back togeather as soon as i started the water it filled up from the bottom of the u-bend and back into the sink. Have i put this back incorrectly? It only seems to go on one way, how can i rectify this?

Thanks

Chris