I think we may have got lost in translation, the bag that was changed is fine, it's not leaking and the sytem is airtight. Soapy water mix used, distinct lack of bubbles.
I'm thinking of the sensor being the problem area, as if the bag was leaking then surely the compressor would be running constantly wouldn't it? I think maybe the sensor isn't telling the compressor the right thing, saying the airbag is full when it's actually not? Opening the valve when it thinks the air bag is too full, thus causing the thing to deflate?
I've been on the Landyzone site also, there seems to be some confusion as to weather a new sensor would indeed require to be calibrated, with a couple of opposing schools of thought. One saying it will need to be calibrated using nanocom etc, the other saying if the neg battrey lead is removed, and the sensor fitted whilst vehicle is at the correct height then no calibration needed?!
I'm thinking coils........