x-ray wrote:
It turns out it was the battery after all. Even though it sounded fine when turning the starter motor over and the tester said it was ok, it seems the vito's are very sensitive to the Volts/amps being used. If it's not correct the ecu won't send the signal to the pump. Why it sent it after several attempts at starting I have no idea, I would have thought the battery would have been getting weaker.
It could have been worse, the guy on the Merc forum spent almost a grand on a new pump and common rail. All it cost me was £90 for a new battery with a three year guarantee.
My motor was the same mate, it turned over one morning but wouldn't start for love nor money. RAC came out and tested the battery and said it's a dud one. Got one from the Auto Factors and problem solved.