My aim in the post was solely to pick up two loose ends from this very interesting thread. The post about brigade headquarters didn't seem to have been followed through and the request early on for a map showing the location of the Whitby Abbey had also not been covered although beach W had been defined. I hope the post answered these two.
The naval despatch is indeed quite clear and Capt Townsend is also clearly named as Beach Master W Beach in the 29th Division Operation Order for the landing. 86 Brigade HQ were temporary passengers (they were not " commanders of the battle") on the Whitby Abbey so it is perhaps unsurprising that the diary confuses the role of Capt Townsend. (It could be said that "under the command of" was referring to his role as Beach Master).
As to the captain of the vessel, presumably James Whyte Harris is the officer who served from 1898 retired November 1913 and shown in the April 1915 Navy List as a Lieutenant Commander RNR Retired - did he return to active service to command a minesweeper?
MaxD