Originally Posted by Dan35
One thing I've forgotten to mention is that the last time I saw her (17 August), she took a cheeky jab at me.
Somehow we got on to the idea of me meeting someone else in future, and she said, "Well when you meet someone else, hopefully you won't be as horrible to her, and then you can...[rest of conversation]." Or words to that effect anyway.

I didn't pick her up on it, and instead let it slide as I couldn't be bothered to argue, but did give her a look of "that was unnecessary."

If she says something similar on Sunday, I was going to establish a boundary (is that the right term?) and simply say, "we're here to discuss the house and I'm here to collect my mail. That comment is not necessary and uncalled for." Advisable? Or is that too aggressive?
Validate. Don't get defensive. WAS's are incredibly skilled at jabbing where it hurts the most.

W: "blah blah hopefully you won't be as horrible to her blah blah"
Dan: "You are saying I was horrible to you."
W: "Yes! What? You don't think you were horrible to me?"
Dan: "It sounds like that felt really awful and upsetting. Can we please discuss the house now?"

Also, saying "That comment is not necessary" is not a boundary. A boundary would be saying, "If you continue to talk to me in that manner, I will cut off our conversation and leave." Then next time she does it you follow through.

You can't control other people, you can only set boundaries on what you will and will not accept, and take actions to show that the boundary is really there (and not an empty NG threat).