I've split from my soon to be ex wife over a year ago,we have now agreed on a settlement sum of 9 grand for me to buy her out of the property which she excepted. She has now already received 2 grand from the settlement which was paid through my solicito, my ex wife is now demanding the rest of the money and is thretening to move back into the property if I do not pay her the rest of the money on the day of the signing can she do this

It depends on how the settlement agreement was written. Those written terms control the situation.

Why would she walk away if she hasn't been paid the agreed upon sum?