Thanks for the detailed and interesting reply. Of course what you
say is very true if the user was selecting only one postcode.
However in the form I am creating the requirement is for the user to
be able to select many, if not all the postcode districts in the UK.
We are going down to the district level, e.g. FY12. Currently we have
Country -> Region -> Postcode Area -> Postcode district in our tree
(e.g England, North West, Blackpool, FY12). So a user can click NW
and then all the sub postcodes are checked below it. For what I have
found most users will be choosing around half the choices, so I am
unable to do reverese/negative selections.
They need to able to do this so the user can choose which postcodes
should be covered. (We are not sending out unsolicited mail in case
you were worried).