Welcome aboard.

This is an old thread, but probably as good as any, given that you have a related issue.

If you've only had the car a week, take it back and get Arnold Clark to sort it.

Most likely, the selector linkage needs adjusting. Unlikely to be the gearbox itself, but still possible.

It should be covered by whatever warranty you got from AC. But, if they quibble, quote the Sale of Goods Act.