Hello Peter,
Maybe when cold the turbo is slightly seized and the fault code comes back, But when hot the turbo has freed up hense no code.
Have you tried running a bottle of Revive solution from Sykes Pickavant through the turbo system to try and free up the vanes?
I'm not saying it's going to make any difference,And the customer needs to pay for this with no guarantee But I have had some good results with this product on various VAG and Land Rovers etc.

Ian