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I'm not sure fred, despite enjoying his portrayal for many years.
I think it is something to do with Cameron having a rather rubbery, bland sort of face, without any immediately obvious characteristics that cartoonists normally find and exaggerate, like Maggie and Blair's mad look, Major's underpants worn on the outside, Boris's wurzel-like, uncontrollable thatch, etc.
In the first few days after the election of 2010, Bell was drawing Clegg as a kind-of waiter, or footman, to Cameron's more important figure, appearing behind him with the drinks tray. I seem to remember an early one of the two, where all you can see of Clegg is a pair of legs sticking out of Cameron's bottom !
Bell has drawn some wickedly funny cartoons of the Queen recently, which I am reluctant to link here, lest the Beefeaters appear at my front door, with instructions to throw me into the Tower. However, they are easily found on the Guardian website, as are every cartoon that Bell has ever done for the paper. His portrayal of Prince Phillip are priceless.
Bell would seem to be the true inheritor of Spitting Image and higher praise is difficult to imagine.