Hi,
In the end, it was a vacuum issue. The only parts that were replaced were a vacuum hose and a breather valve and it has been fine ever since.
I am told that the EA113 engine is very sensitive to vacuum losses (as can be seen by the number of threads on this forum
) which can throw all manner of fault codes. Given the age of our cars hoses, pipes and connectors will inevitably be perishing and leaking vacuum.
The only thing I would suggest is to find a good independent who is experienced with the Mk5 engine and doesn't necessarily go directly to replacing the parts that are necessarily throwing the error codes i.e. throttle body in our case as they may not actually be the cause of the issue but just a symptom.
Even though they are VAG specialists, my indie took them a couple of visits before they diagnosed and rectified it. Thankfully they do not do work unnecessarily even when provided lots of pointers and guidance from armchair car mechanics who read a lot on internet forums like me.
Case in point, my front suspension started squeaking badly recently. I would have put money on it being top mounts, broken front springs, steering rack or subframe. In the end, the front lower ball joints needed replacement.
Anyway, good luck with finding the fault with yours and let us know how you get on.
cheers,
John