Hi
I am sure that someone with more expertise will be along shortly - I have found mention in the London Gazette Supplement dated 28 June 1940 of the award of the DSM to Chief Engine Room Artificer Alexander. Johnson, P/M.2735I, H.M.S. Taku. From a quick Google search, it appears that HMS Taku attacked a German convoy with ten torpedoes and torpedoes and damages the German torpedo boat Möwe on 8 May 1940. After the attack Taku was depth charged and hunted for over 6 hours. Maybe the DSM was awarded in connection with this?
Purp72