Hi Co on left bank only on 3.2 X308 2001

[Kenxj8]

I have X308 that is driving me nuts on an emission issue.
The Right hand bank (exhaust) reads perfectly within all emission specs but the left bank(exhaust) fails on all aspects.
The Co is around 1.8%, lambda 9.67, HC 15%. Should be <0.5 % CO, >9.7 Lambda, HC <14% ?? I am using an Innovate 3000 MOT tester running at tick over and 2500 RPM with only a small variance
The car has had very little use over the past couple of years and historically had no MOT issues with emissions. However, a change of MOT test facility through up the Hi Co failure. After much messing around I realised that the test centre I originally had used was checking on the right exhaust but the "new" test centre was using the Left!.
I have looked at the fuel delivery feedback system which appears to be functioning but consistently resulting in the Hi Co.
I have swapped the front O2 sensors with no change in the results. There are no fault codes present.
My conclusion was that there must be an air leak around the manifold causing the sensor to believe there is a weak mixture, so requesting more fuel..... hi CO results.
I found a crack in a weld at the front of the L/H cat but that has now been fixed with a new weld. Result, no change in emissions.
Any ideas as how to progress to resolve the issue? I really want to identify the problem but not by just swapping out bits.
Am I way out considering a possible air leak, How can I identify and prove it?