re: RNS 510, yes maps can be updated. £200 from a VW dealer, or £100 for same thing off ebay.
re: firmware and map updates solving postcode issue, thats a negative at the mo. as there is someone over on vwnavi.com who has the latest headunit in a brand new car, with the latest v1000 firmware, only just released, and the dealer could not solve the problem. we will have to wait maybe 6 months for another firmware update and that might solve it.
I have the latest maps, and that doesn't solve it either.