Hi Jerome, thanks for your reply.
I've looked into it and thought that it might have been because I seem unable to select a County because for some reason I couldn't use the dropdown in the back end.
I've however removed the County from the back end listing from the Custom fields menu and, despite making sure all fields are entered, I am still finding it won't save.
Regards,
Matthew
PS, I've also found out it if I open the Addresses popup in the User page, make some changes, click OK and then close the box, it doesn't save my updates either.