There is no substitute for professional legal advice about the particular circumstances.
See babybarrister's post above and google Xhydias v Xhydias. IT doesn't matter whether the decree nisi has been granted or a consent has been signed/approved by the court. If there was full disclosure, both parties had the opportunity to take legal advice and the agreement is "fair" (complies with the law) it carries considerable weight. Even though a separation agreement isn't legal binding or enforceable a court can make a legally binding and enforceable court order in the same terms of the agreement so it may not be changed.