Thanks for you help. I'm using 2010 too. I'm confused. I've had a play now and you are right that it does work if put ="US" as the first entry in a given field. You can then even change entrys as required.
However, if there are existing entries in the field in previous rows putting ="US" in the next blank record and expecting it to carry forwards doesn't work. Unfortunately, that is what I am trying to do My table already has loads of entries in it. I am going to try just cutting the rows out, putting in the default formula and then pasting the old entries back in... but there must be a better way?
I also wanted to auto number each new entry uniquely as Access would do. I can do OFFSET() and ROW() variations but these all change when you sort by different columns. Any thoughts?