Thank you AviatorDown8227, your post was the most useful help. I just completed successfully the same failed flight I made yesterday from VTBS to WSSS with the A310 and I had my WSSS destination addon scenery.
I believe this post from AIG was the trigger point. I did not tick this box in the AIM to disable services but I did untick the services in the AIGTC, that should have been the other way around as you indicated. So my heartfelt thanks for your help and solution.