I feel rather stupid now as I had almost written the answer in the question.
I spent some time and read the tutorial on secret menus and found that on the 3490 you can clear all the user data via the trip computer center, if you hold it for 6 or so seconds, it takes you back to having to set up your Local country preference. in my case the UK.
It now works the way I knew it should .
Hope this helps the general pool of information that is amassed on this site.