I think you need to retain a lawyer and get it sorted out. Negative equity when she left and she never paid anything after? Sound to me like a bit of you'll get nothing and like it is in store for her.
The question is, what does she think she is entitled to? If nothing, you have no worries - for the moment. Not sure of the legalities there but In Canada, unless you have a legal agreement, she could change her mind - and it could cost you.
If she thinks she is entitled to more than you agree to, then a lawyer really is the only solution I'm afraid.
Plenty in employment law, not so much in family law :-(
I still have a couple of aces up my sleeve but I'd be a complete bastard if I used them. Having a sit down on Wednesday to see if we can sort something out.
Roll up your sleeves and play the aces - unless she prior agrees to be excluded from any and all proceeds from the sale of the flat due to not contributing for the time frame you mentioned. Get her to sign this agreement and have someone witness it. You can probably find something that fits somewhere on the 'net and make sure to include that the property was in negative equity when you split, that you paid to get out of that negative situation and have paid all monthly obligations since. Get her to sign that she did not contribute any monies from the time you split to present and that she agrees she is not now nor will ever be in the future entitled to any proceeds from your former union, in [real] property, possesions, finances or investments. Make sure it would be legally binding.
I am not a lawyer but I did litigate successfully a labor issue with helpful advice from a friend. I had no other representative with me but did well. The only difference between you and a lawyer is knowledge of the law. If you don't know it and are unwilling or unable to research yourself, get a lawyer. It is a vast undertaking but can be done.
Take it from someone who has been more than fair with another: they don't appreciate it and will turn on you in a heartbeat if they think they can profit. Do not convince yourself that "they will never do that", because they almost always do. I hate legal shit.