I would normally use a many-to-many table but didn't in this instance because of a number of other factors.
Plus, the way WA Search works it was clear that having a comma-separated field would be fine, but it's clearly not working as I expected.
Are you saying I can't do what I need to? Is the only solution to make all the one-character post code areas into two characters (adding a zero for example)?
Thanks for your speedy reply.