House equity share if partner moves out?

me and my partner split in january.she moved out and im paying for the mortgage for me and our two teenage sons.the equity share for her now is �18000.we are leaving the house alone until boys finish school in 5 years.then we sell house or i pay her off.my question is.i will spend the next five years paying the mortgage on my own so will pay off a large share of the mortgage.this will increase the equity in the house.she wanst her equity payout to be based on this figure rather than the �18000 now.can she do this even though she would have paid no mortgage for 5 years ?thanks