As far as not extending properly, I’d check the air pressure. I’d also unscrew the collar with the seal on it and add some grease like Slick Honey and see if that gets things moving again.
As far as the post dropping when you sit on it, I haven’t come across that with mine. I guess that it might be due to the hydraulic valve staying open a little bit; maybe due to a sticky remote cable. One test would be to remove the remote cable at connector toggle (little rectangular box) and see if it still drops. The dropper cable and housing are just standard derailleur stuff.
I’m in the US, and KS USA has been no quibble as far as warranty; I have no experience with the UK.
Here is a video on servicing the Lev [video]http://www.youtube.com/watch?v=SOYjhtPFZ4I&feature=youtu.be[/video]