OK, so I agonised over this lots, so thought I ought to share it.
If you have changed region the 4 times allowed on your mac, and try to change for the last time on a European disc, then the chances are, you'll get a 'Error -700001'. This is annoying, and worrying.
There is a simple reason. If the disc you're using is Region 2 + 4 then this will happen. If it's just region 2, then you'll be fine.
Fix: Use a single-region disc to reset the region on your mac. Sorted.