I think (thanks to VCDS) that I have identified (one of) the problem with my A8.

Perhaps prematurely, I bough a second hand TCM as I believed that it would be a part number swap (4D0 927 156) however I have noticed that the last two letters don't match - is there any relevance to these letters?

In any case, put it in, car went into limp mode and complained that it was improperly coded - tried changing soft coding but it just won't let me. (I could do this as much as I liked with the old TCM - is this where I would need a login?)

Is there something else that can be done using ross-tech or am I going to be forced to hand over loads of cash for a new one and possibly a dealer coding.

Any help appreciated here.