first up thanks (not) to the support staff @ microsoft and also on the forums here for (lack of ) help on download content etc and this error message.
gave up entirely phoning microsoft as all they kept telling me to do was the steps that are online - reset cache /delete stuff off xbox hdd etc.
lo and behold today i did actually manage to finally purchase an online pass for SHIFT2 unleashed
what i had to do:
1>log in to xbox.com
2>buy points (using PC NOT xbox360)
3>search the marketplace for SHIFT2
4>purchase the content (which adds it to download queue on xbox360)
Unlike some I WAS able to purchase points using my 360 however was unable to purchase this specific content - I could and have bought content since I had the original (and still continuing if using xbox360 to try and purchase) error.
Seems like there is obviously more than just 1 or 2 problems with the current marketplace system, no matter where the user lives.
I have seen some posts where people were lucky enough to be given a redeem code by EA since they were unable to purchase content, however not so easy in the UK when all they do is refer you to Microsoft - who stick you on a permanent roundabout of reset/delete/wait 72hrs, rinse/repeat...
Hope this is of some help - god knows there's not much in the way of solultions either on forums OR from Microsoft .
Have to say I am extremely disappointed it has taken me a full month to be able to reach a satisfactory conclusion - Yes, I could have just went and bought a new copy of the game, but that still doesnt address the original problem!