Just returned home from work to find your posts Lucy. I could kiss you. flipping over all this new info for me.... Your a beaut! You too KHP. that proves to me he was in the navy , and it does look like it was with the marine engineers, so the wellington link may have some truth to it possibly.
Also found this.....below. so It seems he was still with the Royal Industrial marines on his way back from India in 1924, then headed off to New Zealand.
UK Incoming Passenger Lists, 1878-1960 about Whilliam C C Cook
Name: Whilliam C C Cook
Birth Date: abt 1901
Age: 23
Port of Departure: Bombay, India
Arrival Date: 12 Jun 1924
Port of Arrival: London, England
Ship Name: Caledonia
Search Ship Database: View the 'Caledonia' in the 'Passenger Ships and Images' database
Shipping Line: Peninsular & Orient Steam Navigation Company
Official Number: 102390