ADVERTISING agency Backer Spielvogel Bates was last week practically biting off one of the hands that feeds it in its Media Guide. The agency-produced handbook gives newspaper and magazine circulation figures, readership profiles and advertising costs. But the authors pull no punches with television stations TVB and ATV, once they make it clear the medium accounts for half the advertising pie. Ratings have declined by more than 30 per cent in little more than five years, and one of the reasons is the lack of quality programmes, according to the guide. ''Observers . . . must have noticed that people are talking less about TV programmes these days [except how lousy they are], fewer people are rushing home to catch a TV special, and many don't care what's on the screen now.'' Production values are ''mediocre'', it says, and ''television stations should instead try regaining their audiences with improved programming and better production values''. The vitriol runs to the last: ''Instead of maintaining revenue with double-digit percentage rate hikes, they should be working to reassure advertisers that TV can still be a cost-efficient and effective advertising medium.''