Welcome to the forum and thank you for the photos.
The Navy List for June 1944 lists Sedulous with Chief Skipper D Main RNR, and also lists Doonie Brae with Skipper H A Hodds RNR. Reflect and Tweenways are also listed but have no officers shown. One reference on the web states Doonie Brae was a Minesweeping Trawler, so it’s possible the others were too. Elfin was a shore base for submarines at Blyth, and Elfin II the Coastal Forces base.
ML 256 is shown in the July 1945 Navy List as having Temp Sub Lieutenant V. H. Ambler along with your father. This boat was part of the 17th ML Flotilla and had been on station with the rest of the flotilla at Takoradi by March 1942, so it's likely your father would have booked passage on another ship and joined the boat at Freetown.
The situation with regards to ML 356 [Edit: Boat details now ammended above] is a little bit confused. That boat appears to have been built by the Anglo American Nile Tourist Company, Cairo, and was presumably based in the Mediterranean theatre. The 23rd ML Flotilla is known to have comprised MLs 291, 309, 343, 344, 346 and 464, and one source I have states Lt Alex D Fear was Commanding Officer of ML 346 which was at Dieppe. Other MLs recorded for Dieppe were 291, 292, 309, 343 and 344, all of the 23rd, along with elements of the 2nd, 4th, 11th and 20th ML Flotillas.
I'll drop you an email if you don't mind as I would be very interested in the materials you have collated about the Dieppe Raid.