The Guardian report of Mattarosa and the Times report of Treeve on the GY-1151 Mercury is validated by U-boat.net, which identifies the submarine as the U-25, under the command of Kapitanleutnant Otto Wunsche. The attack took place on 9 Jun 1915, and the U-25 is credited with the sinking of Castor (182 t), Tunisian (211 t), and Cardiff (163 t). Velocity (186 t) is listed as being sunk by the U-25 on 7 Jun.
The UC-16 was under command of Oberleutnant Zur See Egon von Werner when it sank the GY-1151 Mercury about a year later.
As for the Hull boat, U-41 under Kapitanleutnant Claus Hansen is credited with sinking the H-518 Mercury (222 t) on 2 May 1915.