GCap Media's London music station, Capital Radio, has revealed it will dispatch people armed with lipsticks and thousands of innuendo-laden, hand-written notes across the city next week in its first advertising campaign for 18 months.

Britain's biggest commercial radio group had held back on promotions for Capital as it streamlined the business following a £711m merger of it with GWR last year. But the struggling media group said mistakes that lost Capital the title of most popular London station were now behind it and it was time to claw back listeners.

The campaign, starting on Monday, is based on the slogan "Who's doing who?". Posters, scribbled notes, web adverts and faxes to London firms will ask questions like "Is Christina doing Robbie?". If Capital is right, consumers intrigued by the cryptic questions will tune in to hear a competition where listeners guess which artist is singing another's song.

"There's a feeling that over the last three or four years Capital has retreated a bit from that sort of street level activity and has become a bit more remote and detached. We have to re-engage with people," said Steve Orchard, operations director at GCap.

Capital's advertising team will also scrawl questions like "Is Lily doing Justin?" in lipstick across 5,000 London pub mirrors. The competition will dole out £100,000 in prize money, but the station is tight-lipped on the campaign's cost, estimated by analysts at £1m.