What is wrong with my Vauxhall Frontera?

I recently bought a Frontera 2.2 DTI. When I eventually reach a speed of around 55-65 MPH an orange warning light situated on the rev counter dial shaped like an engine lights up. I've looked in the handbook and there is a symbol similar to the one that apears but with the addition of a bolt of lightening through it. The the car seems to be running ok, and when I slow down again it goes off. I find it disconcerting. Anyone had a similar problem or know what's causing it?