Discovery 2 TD5 Suspension Issue

Hi,
I am looking for a bit of advice as to what the issue may be with the rear suspension on my 2001 Disco 2 TD5.
It is still as it was when it left the factory, no modifications so it is still on air.
Had it since February this year and its been totally fine, then a couple of months ago the drivers side rear suspension dropped slightly after being parked overnight.
Had it at the garage and they found a small hole in the airbag so that was replaced. All was well for 24 hours then it dropped again, right down, on the same side. They had it back in the garage and tested for leaks and couldnt see any pipework issues or trouble with the new airbag or anything else.
They suggested maybe to replace the ride height sensors but said they couldnt guarantee it would solve the issue.
I have just put up with it for the past few weeks but want to get it sorted out properly really.
It re-inflates fine in a few minutes when I turn the engine on and then stays up for several hours after its been driven. Only drops totally when its been stood for longer like overnight.
Does anyone have any suggestions please?
Not necessarily able to do the work myself so I just need a clear plan to give to garage of what I want doing.
Thanks in advance!

Wishing to not state the obvious but you have a leak..

Either at the pipework connections or the valveblock "located in the black box on the side of the NS chassis rail"

Some soapy water and a brush will get ya started.

Check the valves in the compressor box, one may be sticky

Definitely check valve block, if it’s had a new bag shouldn’t Realy be that, ride height sensors do just that, detect it’s height/ level, if it pumps up in a morning then cuts off sensor is knowing that side is low then telling compressor to pump then once level tells it to cut off.

Valve block is just at the side if the compressor within it’s black box, only 1 pipe feeds each bag and they just screw in to the valve block, this is plastic by looks of mine so go easy with connections, would think they strip very easily. Had to undo mine other day as passenger side sensor failed and well over inflated the bag so undo connection on valve to drop pressure.