Microsoft / Nokia are rolling out updated offline maps for Windows Phone 8. The update which seems to be available for all Windows Phone 8 devices includes the latest map data and is available for offline mapping. On my machine the update for the UK came in at just over 200mb.
You can manually update the maps by going to settings in the maps app and then pressing on Check for updates.
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RT @isdixon: Offline maps for Windows Phone 8 updates: http://t.co/wtZbe23cUV