Is this one from 'Bibi' Netanyahu. The idea is that he turns up at the babysitter/Bibisitter one evening. It is nicely judged and even funny (watch it to the end).
It struck me that it's a great way to remind people that a conservative politician is a serious grown-up while simultaneously showing him to be a human being. If Cameron has the lightness of touch to pull it off (I doubt it - but I wouldn't have had Netanyahu down for it either) then he should do something similar, perhaps with Cameron having to undertake some equivalently normal but important task. Here's the script:
Cameron: It's either me or Ed.
Normal people: But Ed will forget, like he did with the deficit.
Cameron: Or it could be Nick?
Normal people: No, he'll say he'll do it, but he won't, like with student fees.
Cameron: Or Nigel?
All three: (laugh)