I think I know the answer but just after a quick consensus.   The car works hard covering 30,000 miles a year, I'm not meticulous with washing the car but do keep on top of it every three or four weeks.   Doing the mileage I do means the car picks up it's fair share of stone chips which I always touch-up as soon as they are spotted.   However when washing the car today I noticed quite a large patch of corrosion in between the grill and the leading edge of the bonnet