Yes unreasonable behaviour can be used I would think.
One piece
of advice I was given was really useful- that was- keep the reasons
relevant and mention your feelings about it: someone abandoning you IS
relevant- ie- your first reason can easily be something along the lines of
"they left me x amount of time ago, which made me feel x way".
Though my wife had left and taken up with another guy, I was told several
times not to divorce on
adultery since if defended it would be hard to defend,
but when I sent to the court on unreasonable behaviour, I mentioned an
"inappropriate relationship" which made me feel "x and y" way.
I
followed that up with other relevant information and how I felt about those
things.
Bare in mind- irrelevant or petty reasons for
unreasonable behaviour can also be thrown out of court, because they're not
unreasonable enough- but abandoning you is a reason why your marriage has
ended.
Hope that helps- and I'm no expert!!!