If it won't go up at all, on either side, it sounds like your compressor has packed up - Is your Disco a 7 seater?. If it isn't, consider a coil spring replacement for about £80, better than the £100's to replace the compressor. If it is 7 seats - LR didn't get the disco approved with coils springs for the 7 seater, so best to stick with Air.
I had a similiar decision to make - Glad I stuck with Air, despite the hassle and expense - Remember to check the shocks as well. Mine were shot when I did my Air springs.