MrsCheeks has misunderstood the press release she linked to. Here's the straight answer: Ronnie Hazlehurst wrote the original theme (in fact, two themes, an opening theme and a closing theme, both instrumental). The original theme was used for the entire first series. Then John Sullivan managed to persude the BBC to change it. He wrote and performed the new opening and closing themes. He wanted Chas and Dave to do it, but they were busy as their career had just taken off thanks to "Rabbit" and all those other highly memorable tunes that I can't remember anymore, hence he sang them himself. Later on, the BBC replaced the original themes on the first series with the new themes, EXCEPT for the first episode, "Big Brother", which for some reason they never bothered re-editing and which still gets repeated with the original, and frankly rubbish, Ronnie Hazlehurst themes.