thanks for the advice guys.
i tried the stress bonding technique with them last night. we brought them in separate cages to a friend’s place and then released them in the new neutral space of the friend’s living room. they ignored each other mostly, which was a marked improvement! after 20 minutes they went back in their cages and i left them side by side for another 20 minutes. again, they were quiet, whereas at home they’d still be trying to attack each other through the cages.
i’ll keep this up over the next couple of weeks and see how it goes.
when they were first introduced there were no problems and they bonded pretty much immediately. bessie was very little then so i suppose colin didn’t find him any kind of threat, and bessie was used to being around other rabbits.
i really hope they can be friends again. colin is happy hanging out with people, but bessie seems all depressed since the fall-out.