Hi Tramroads
You defo need to take it back to the garage (if you trust them) as you've commissioned them to repair your Disco, and they've clearly failed.
By taking your money and declaring the air bag issue resolved - they've a duty of care under the Sale of Goods Act to complete the work to a reasonable standard and to have actually completed what they said they were going to complete.
You've got two routes now - under the law, you need to raise a complaint with them and give them the opportunity to actually complete the work to a reasonable and competent standard.
If they don't wish to do this - hopefully you've paid this £300 with a credit card? Ifso, get onto your card issuer and report the issuer to start a charge-back case against them.
Final route is County Court claims if they continue to either fail to complete it, and/or refuse to take it back to complete it. Obviously, this is a reasonable amount of hassle but not as much as you think as you can start it online these days. Tell the garage that hasn't fixed your car that this is what you'll have to do if they don't get your car sorted!
Sadly, I'm in Kent and couldn't be much further away from you in terms of actual experience of local mechanics to you. However, I've heard some reasonably good things about:
Welcome to the Land Rover Centre Huddersfield web site - We buy and sell new and used land rover, Range Rover, and Freelander, series and Discovery models - for sale - which are in Huddersfield. (Really hope these aren't the guys that have messed you about thus far!!)
Hopefully, one or two other forum members who live nearer to you can make an alternative suggestion? You're in Land Rover country up there, so there must be loads of good guys that know what they're doing?!
Failing that, I'd consider taking it into the local main dealer to have a single hour's full diagnostic on their Testbook computer - for this, expect to receive a comprehensive report of every system on your Disco - they should be able to report to you any stored or current faults on your motor. This process actually only takes about 30 minutes, so ask them to spend the other 30 diagnosing the remaining fault with your air suspension... However, any local LR specialist that is main dealer trained, should really be able to get this sorted for you without any real hassle - the air suspension system is really very simple indeed.
PS - it's good practise to replace the height sensors in pairs when they fail. Did they do this? Did they recalibrate your system after the sensors/bag were replaced?
Finally, there isn't a "ride height sensor" light on the dash - did you mean the yellow light, which is a rear view picture of the Disco, with an arrow on either side implying side to side wobble? Or was it the green one with the side profile of the Disco with the "up" arrow at the rear?
PPS - if both sensors have been replaced and your system is dropping down - this sounds like there's an air leak from your system still. If they only replaced the one bag, it's entirely possible that the other is holed. I have just had to resolve an air suspension issue myself, where the bags seemed to fine, always rode at the right height, hitting the off-road button would raise the rear end properly - however, when the engine was off, and you got into the back of the car (the kids for example getting into the car before we did and started the engine) within about 5/6 minutes, the car would have dropped down onto it's bump stops.
The best test, is to get a squirter bottle full of soapy water, hit the off-road button to expose as much of the bag as possible, and to cover the bags in soapy water and look for bubbling anywhere. Don't expect to find a hole that you can see, but you're in effect trying to do a puncture test as you would on a bicycle inner tube on the bag whilst it's in situ... The hole on mine was entirely invisible, but was there sure enough and replacing the bag cured the problem immediately. Final thing is that you need to get this sorted as quickly as possible, as if you have an air-leak, the compressor will be working harder than it should and it will shag it out fairly quickly! The Wabco compressors weren't up to the job really as it was, so air leaks kill them really quickly. You don't want a £400 bill for a brand new air compressor on top of the issue that you already have! Whatever you do - get it sorted as quick as you can....