Bolty,

Marie Susanne is listed in British Vessels Lost at Sea 1914-18 as coming under attack on three occasions in 1917:

On May 22, 1917, she was missed by torpedoes in the Mediterranean
On June 18, 1917, she was "rescued" from a gunfire engagement from a U-boat in the Atlantic
On November 19, 1917, she was in another fire fight with a U-boat in the Atlantic.

Given the date, I would suspect the award was for the November action. I'll see if I can find more details.

Best wishes,
Michael