Hello Ewan
I went to the museum and i am sorry to say there are no marriages for william cox and nothing for Maude cox, i did search the voters books for 1899 up to 1909 and did not find him there, i was told however because he lived in union mills the address in douglas would only be his place of work, looked at his buriel and there was no stone, but it is william henry cox age 77 as you say, How sure are you that this is your william that was over here? lastly i had a look in the death colume in newspaper for 1910 this is what it said,
Cox-on the 1st November at Ballagarey cottage Marown William Henry Cox, silver plater late of Sheffield age 77 years-interved in Marown church yard on the 4th, there is no mention of any family or any one else, if you think it will be worth it , do you want me to Pay £3 and go to bmd and look at death cert see what it will say, but i cant go untill wednesday,