my peugeot 2.5 boxer wont fire up?

i have a 2.5 peugeot td boxer on a compass camper box, I have only owned it for a couple of weeks, got up one morning and the engine wont fire up, it turns over ok, ive checked the fuel at the injectors, that seems ok, but i have noticed that the pre heat light on the dash only comes on for about one second, then goes off, so i put a volt meter on the glow plugs at the engine and the volts only go up too around eight volts for approx one second, could this be the trouble?