Well it looks like its fixed and the answer was in the posts above. I had ran out of ideas on this one so despite the fact the APPS earth tested OK I re checked all the earth points I could find around the car. They all looked clean and good. I then physically removed and checked them, Again all appeared fine except for a multi earth point below the battery which was slightly corroded but not bad. I cleaned it up and sent the car back to customer. The car was in last week and when I asked the customer he said its been faultless.

I assume that the earth point must be an ECU earth of some form that has caused an issue and made the ECU flag the APP code.

Many thanks for the help.
Steve.