The banana does slow the monkey down, although it's partially because it smacks it in it's face, and that's rather irritating. It slows down so it can scoop it up and crush it in one hand, but then hesitates and brings it up for a sniff.

There's a faint hint of an interesting smell to it, a touch of something the monkey thinks it would like to eat. So it bites down, not bothering to peel the fruit first. A few seconds later, though, the monkey is tossing the banana away in disgust. Whatever it thought it was supposed to taste like, it clearly doesn't.

It picks up speed again, going after the things it knows will taste good, but the diversion bought Jamie and the Doctor enough time that they should be able to get through the door at least.