Written out, Tolwyn (in any deployment record or such) would be:
"Rear Admiral Sir Geoffrey Tolwyn, K.C.B."
Ranks always come before anything else, peerages included.
Examples being:
"Captain Lord Thomas Cochrane, Earl of Dundonald"
"Vice Admiral Lord Horatio Nelson, Viscount Nelson of the Nile"
When written with last name, it would always be: "Rank Lord Name" and form of address when refering to them as first person would be "My lord." "Your lordship" rather than "sir."
Also, with knighthoods, they're always referred to either as "Sir Firstname" or "Sir Fullname" never "Sir Lastname." So Tolwyn would be "Sir Geoffrey" or "Sir Geoffrey Tolwyn" but never "Sir Tolwyn".