In reply to Simon Caldwell:
OruxMaps used to use Bing maps as a tile server for OS mapping. Then the OS set the heavies on the guy who develops OruxMaps (a free app), and forced him to remove Bing OS mapping as an option, and to install a block to prevent people adding the source manually. So OS mapping disappeared from anything later than V5.5.3.
I kept an old copy of V5.5.3, as it still allowed me to get OS mapping from Bing. But I tried it recently, and it no longer does, so I guess Microsoft have modified the API for BingMaps to prevent it serving tiles to old apps. Which is a shame. I guess I'll just have to dust off Mobac and create tile sets for OruxMaps...