The correct form of address for the Lord-Lieutenant is:
Written on the envelope: Mrs Susie Sheldon JP, HM Lord-Lieutenant of the Isle of Wight
Salutation in the letter: “Dear Lord-Lieutenant”
In a formal speech: in the preamble “My Lord-Lieutenant”
As a helpful guide it should be noted that the Lord-Lieutenant is Her Majesty’s representative in the County and should be afforded the same courtesies given to members of the Royal Family or dignitaries when they attend an event on the Island. The same protocol is in place if they are represented by the Vice Lord-Lieutenant or a Deputy Lieutenant.