Unless you know for sure the glow plugs are knackered, leave well alone, as P@f says, if any snap, then it is BIG money, as the whole cylinder head needs to come off. On my old Berlingo, it was a doddle, as they were at front of the engine, easy to get at, on the current van, a mk2 Dispatch, they are hidden at the back of the engine and very difficult to get at. Been told to leave well alone.