Idling prob on Renault Megane Cabriolet 16V (W, 2000)?

Hi,
Car is idling when stationary, it doesn't cut out and drives perfectly. We've had AA out to us but nothing shows up on diagnostics. We've check all pipes, plugs, map sensor but it makes no difference. Any other suggestions would be helpful to try to sort this prob that's driving us insane!!! Thanks