I have to say very many thanks go to Sotonmate from the 1914-1918 forum who not only told me that a book existed with rolls of honour for all the Elder Dempster ships lost in the Great War but then went and copied the relevant pages for me. I would not have been able to get this information myself so quickly. Sotonmate, you are a star!
I have already posted a copy of the roll of honour for the SS Burutu (relevant because it contains J. Tonkies from the memorial) I was also provided with the chapter detailing the SS Burutu's battle with a submarine and then her accidental collision and sinking. I will post transcripts as soon as I get time. (see page list on the left)
I am confident that I will be able to use this information to add James Tonkies to the Roll of Honour at the Town Hall. It is such a shame that he was not eligible for the CWGC. At least now he will be commemorated in his home town, on the same Roll of Honour as his brother.