UPDATE (Evening): No official announcement/confirmation - looks like the coalition are sticking to announcing the date officially next week (I think they would look bad if they didn't announce it through Parliament). BUT: After Ross Hawkins, (Beeb Political Correspondent) phoned me earlier to try to establish who my source was, I just saw James Landale (Beeb Political Correspondent) confirm that the BBC believes that the LFF story that I broke above is correct: That the government will announce next week that the referendum will be held on May 5 2011.
UPDATE (Morning of 2nd): The story is now the lead on the BBC, and is spreading all over the media. My story was right, and the coalition may have been forced to bring forward its tacit confirmation of it as a result of my breaking the news. :-)