SO farewell then, Teresa Norton - the broadcaster's honeyed tones were missing from the FM Select airwaves as of this morning (there is much speculation as to whether she left of her own accord - or, er . . . was pushed). Official word is that Teresa was departing ''to pursue other interests''. By coincidence, John Nielsen, manager of the Grand Hyatt's Champagne Bar, told us yesterday that he was having an affair with Ms Norton. As we raised our eyebrows, John hurriedly pointed out that he meant ''a musical affair'', at the Champagne Bar on the next five Thursdays. The usually talkative ''Mother'' Teresa, however, remained tight-lipped, apart from sending us a note saying she planned ''on having some fun . . . but Mumm's the word''. Has she really taken a vow of silence - or could she be referring to the champagne company that will sponsor her musical soiree?