So it appears that my car has started to sag again on the left hand side despite having swapped both airbags.

Parked on drive & I measured the distance from bottom of wheel to wheel arch trim and it was 727mm when I came out in the morning it had dropped to 722mm.

When I started the car the compressor came on and pumped up for approx. 30 seconds

I've even checked how level my driveway is to see if the car is levelling itself.

Only thing I think it could be now is compressor or valve block..