RUPERT MURDOCH is making a sustained and expensive bid in the United States to increase audience levels for his Fox TV network, to rival those of the big three - CBS, ABC and NBC. At present Fox has only 12 per cent of the average weekday prime-time audience - 6 percentage points lower than NBC, the weakest of the other three. But last week Murdoch paid dollars 500m ( pounds 330m) for a 20 per cent stake in a group of 12 stations that will switch to Fox - mostly from the second- placed network, CBS. This follows a dollars 1.6bn deal last September with the National Football League, to air its games nationally, wresting the contract from CBS. The Fox network now has 143 affiliate stations, as against 94 when Murdoch created it in 1986.