Thanks so much for your help Stan and seaweed.
Stan thank you for the link, I had no idea it was so complex! The Miramar ship index is vast.
Seaweed - thanks! That's very interesting and looks hopeful too, I would love to find out more about James' career, he was a merchant seaman all his life, he died in 1944. My grandmother missed him very much when he was away and was always very excited when she knew he would be coming home.
I'll have to get to the Portsmouth record office somehow! I'm living in Ireland at the moment but I'll try to get as much information as I can and plan a quick trip.

Many thanks again - feel I have some chance of finding James now.