Oh,and as a brief followup: Post code searches actually seem to work from http://www.geocaching.com/seek/nearest.aspx. Which exposes another of my annoyances with gc.com: The site layout is, how shall I put it, less than optimal. To get to the above link you have to do a (broken) postcode search, then click on "New search". Why this isn't there in the first place is a mystery.
Oh well, at least I get to partake in the national pastime of whinging