Your ex is a legal owner of the house - that is why she was informed that you are trying to sell the house.
As an owner she is entitled to a share of the proceeds, however unfair that is. That share may not be half.
You will argue she hasn't paid anything for 20 years; she will argue you have had the benefit (lived in the house) for 20 years.
20 years ago you should have bought her out and got her name removed from the mortgage and the Title.
You need a solicitor.