Another post I'm shamefully copying from Chris Paton's blog (I'm sure he won't mind!):
If you are researching merchant seamen who served in the Second World War then you might be interested to know that an index to Merchant Navy Gallantry awards in series T 335 is now available through The National Archives online catalogue. You can search for awards by the individual seaman or by their ship.
The records show the date the award was gazetted, the medal awarded and the rating of the person at the time as well as the ship he was serving on. However, probably of most use are the citations which give the reasons why the medal was awarded in the first place.