Some people thought it came from this clip: (starts at 1:48, should queue right up to it when you hit play)
which is the one where you hear Benedict say ‘sweetheart’, believing they hear Martin saying ‘sugarnut’ in there somewhere too, but those quotes are actually from two different events. I think the confusion in the video is when Ben says “I just thought you were making sh[bleep] up”, which with the bleep does kinda sound like it might have been ‘sugarnut’ (though someone would have had to mix up who was speaking), and there’s also a bit where Martin’s hard to make out - he says “I thought you’d been christened by him” but if people were determined to hear the sugarnut bit, they might feasibly try to squeeze it in there. Mostly, I think some assumptions were made about it having happened at the same time, and so that’s the information people were passing on. Here’s the transcript:
Ben: …saying sweetheart, I wh’ the [bleep] is that all about? I don’t know.