Who is Tom and Jane Grenville from The Talbot Odyssey and what is their importance?

Tom is a compatriot of Patrick O'Brien's and George Van Dorn's. His wife, Joan, is known to sleep with Tom's younger, more attractive friends. She makes passes at both Tony Abrams and Marc Pembroke. While both seem to be easily dismissed, those who look closely notice deep reserves of strength and courage in both of them.