Thanks but I had not found Debretts useful for what I wanted. Perhaps I need to spend more hours with it.
I found the answer in the Wikipedia link below - Order of Precedence in England and Wales.
The last entry in the table under heading Marquesses et al says - Eldest sons of Dukes, etc
Then follows the heading Earls et al and table.
https://en.wikipedia.org/wiki/Order_of_precedence_in_England_and_Wales
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