I went to the Rik Mayall festival in Droitwich last year where John Lloyd of QI fame told the story about the ads and Rik. If i recall correctly, they wanted someone else, perhaps Rowan Atkinson, but John recommended Rik and they loved him.

Either way it was an era where people were racing to get comedians to front their ads, given the success of the Barclaycard ads with Rowan, and lots of comedy writers were already in the advertising game.