Looks like it is going to cost you money already. The suspension is very suspect unless you're a mug like me and go to LR and pay £1500 for a new compressor and 4 new air bags.
There are all sorts of reasons why it may be down on one side. The valves may be stuck, the bags may be shot. Be strange if both bags on just one side went though.
If you don't mind getting your hands dirty, then this is a damned good site.http://www.rangerovers.net/fixindex.html