It's a W.W.W (watch, wristlet, waterproof) made at the end of the Second World War for the Army. Much more info on MMH's excellent site, http://www.time514.com/BritishX.htm
Without wishing to sound impolite, I hope you didn't pay too much for it. The lume is poor, the second hand is wrong, and the crown is also wrong.
Next thing you know, you'll be looking to collect all 12 WWW's, and all the dial variations...