I have a few British Merchant Mariners on my 'to research' list that are buried here in Holland. Right now I have no idea where to find this kind of information. I assume you exhausted
www.google.com (search-engine). I would think trying the
http://www.pro.gov.uk/ would be a good start. Maybe e-mailing them and asking for the address of the Association of Merchant Marine during WW2, which I am sure exists. At PRO they also should be able to help you to the man's service records or refer you to an agency that will hold these doc's.
Good luck! And let me know how you do, so I can use that knowledge in the future!!! I will be researching the lives of several mercant seamen in the future...