You can imagine the conversation. Chocolate man to wacky adman: “We need some of that guerrilla marketing stuff. Very popular with the youngsters.” Wacky adman: “We'll get straight on to it.”

A month later. Chocolate man (head in hands): “It's a bloody gorilla. Don't you people listen?” Adman (improvising madly): “Sure, it's a gorilla, but it's actually a multi-segment, through-the-line randomistic-viral experiential marketing piece. But in case you're worried, we'll put a purple background on it.”

That may be the story behind the creation of the Cadbury Milk Tray drumming gorilla ad, or it may not. But there is no doubt that the commercial, viewed hundreds of thousands of times on YouTube and on Sky, is the most talked-about bit of publicity of the past 12 months. It was singled out in the reports of Cadbury's annual results in a way that suggested that the hairy percussionist had single-handedly transformed the fortunes of the proud old confectionery outfit.

Cadbury revealed that revenues were up 5 per cent, benefiting, among other things, from the success of the Dairy Milk ad campaign. Is the gorilla shifting chocolate bars? Some people remain sceptical, such as the contributor to the Brand Republic website, who writes: “Dear, oh dear. How can a 5 per cent revenue growth for a company with such a diverse brand portfolio be attributed to one ad?”

It got people talking, however, and that's the only measure of effectiveness that many marketing people need. Expect commercial life to become very random from now on as ad breaks begin to look more like an installation wall at Tate Modern than a man pointing at a box of soap powder and saying: “Buy this - it's good”.

Come to think of it, that might be a really radical idea for a campaign.