speedo works intermittantly?

On our Renault Megane Scenic [s reg] the speedometer works when you first start driving but then stops again after a short distance.Sometimes it will start working again but more often than not it wont then the electrical warning light comes on. Any one got an idea on what could be the problem and how to remedy it?. The speedo is analogue but the mileage is digital

Paul
February 2011