Thanks for the replies. eyeinthesky - a very clear description of the system, thanks. Agree with Dan Dare - there's no need for an airport to input data to code-callsign convert or incorporate issued squawks into an airways clearance. Similarly with the case of the nearby military LARS unit, they won't input any data themselves and have been allowed a feed of the data, even though they're not NATS and therefore 'unaccountable'. I'm also not aware of any money having changed hands in this case.