The Mercantile Navy List is the latest addition to our digital library archive at http://anmm.smedia.com.au/olive/am3/anmm8/
Hard to find and not well known in Australia this is the most comprehensive listing of British registered merchant ships. It’s also a really good place to look for official numbers and signal letters.
The earlier volumes are part register and part directory. Issues up to 1869 have lists of master mariners with certificate numbers. Other useful information to be found in these older volumes include lists of pilots, port and maritime officials and also advertisements.