Air Suspension Problems

I'm sure this has been discussed at some stage but I am new to this site so this is first up for me. Have Disco II TD5 (2001) rear left side keep sitting down when not running, not all the time but randomly. Have had the teeth sucking at the garage without even looking at it and a quote of £250 per side to replace. Does anyone have any information about this and maybe somewhere I can aquire these airbag/springs at a reasonable price? any help would be much appreciated. Thanks

air bags weigh in at about £80 each and as easy as changing a light bulb to replace.

Check these links:
Rear Air Suspension Spring RKB101200

http://www.landyzone.co.uk/lz/f8/td5-air-suspension-strange-goings-47626-3.html?highlight=suspension

search on here been covered a million times...