That looks like a rather nice example of the Lemania single-button chronograph, with a Tritium-lumed dial (thats the T in a circle) and pheon (the broadarrow denoting govenment property) in a series 1 case with the original fixed lugs. If you open the case you'll probably find a gilded (gold coloured) manual wind, unsigned, movement in this earlier example (rhodium plated for the later ones I've seen).
These were issued to various departments of the British armed services between the mid 1940s to mid 1970s. They would be stamped with various codes on the case back which changed over the years. This one is stamped 'A.M' for Air Ministry, '6B' (usually found as 6BB) for the RAF, '551 217' I'm not familiar with but by convention these numbers would refer to the type of watch (chronograph, wristwatch) and '53' would be the year that it was recieved into the RAF stores. Typically, they would be issued to flight crew, particularly navigators.
It would be nice to see a shot of the movement if possible to confrim some of this, and learn if your colleague's grandfather had any military connections.
I think these are cracking watches. At 38mm in diameter they are a good size for a vintage watch and very wearable today. Well worth getting it serviced and hanging on to as a valuable heirloom.