Thanks for the suggestion, but my definition of the correct format for a
phone number is different to Outlooks - I prefer to view it without all the
brackets and the +'s etc.
I was also worried that when I synced the numbers back to my phone it
wouldn't handle the brackets etc, but it didn't seem to mind.

Anyway I figured out that if I just typed in the phone number including
spaces where I want them, Outlook doesn't seem to touch it.