Not sure it was classifiable as a craze really- comedy/novelty songs have always had a market and the fact that a small number of them (two or three?) happened to be sung with cockney accents around the same time isn't particularly significant I feel- in other words I think the comedy content was the primary appeal. I don't think "Fings" had anything to with it personally- we'd had Oliver after all which was all broad London. Actually I've never particularly thought of the Bernard Cribbins ones as cockney until you mentioned it (and I'm a Northerner)- I just thought they were good fun- I do think he's a great singer by the way who should have done much more. Been enjoying your blog incidentally.