Not sure if Det 410 refers to being detached to Landing Craft 410 - from RM Museum info:
Training Flotillas
These initially were all manned by seamen rates, although at Eastney in Portsmouth, there were craft in 1940 manned by Marines for training purposes. In 1943 RN officers and ratings continued to carry out the major part of the crews’ instruction but by the end of World War II there were RM training staffs, with flotillas of various types of landing craft, including the following: 400 to 435 (incl 410), 437 and 490 of 1945. In a typical change over to RM staff, 235 RM LCP Flotilla relieved the training staff of RN Training Flotilla 188 as early as 18 June 1943. Some training flotillas also served in operations as Ancillary flotillas.