Now generally a final order cannot be varied the Court can give directions to implement the order under the liberty to apply provision.
To sucessfully vary you woulkd have to show that the issue of structural problems were never contemplated.....were there any signs then?
Was the property valued in the case....we always tell clients a valuation should be carried out by a chartered surveyor to flush out such problems -never rely on a market appraisal...risk is parties may agree the value.
If that was the case you could argue a mistake and seek a variation under the principles in Barder v Barder.
Warning...must act very quickly after knowledge
and before other party acts on original order to her detriment.
Barder v Barder
In the leading case of Barder v Barder (Caluori Intervening)  AC 20 the House of Lords stated that a court may grant leave to appeal out of time if the following four conditions are met:
1. New events have occurred since the making of the order which invalidate the basis, or fundamental assumption, upon which the order was made, so that, if leave to appeal out of time were … given, the appeal would be certain, or very likely, to succeed.
2. The new events should have occurred within a relatively short time of the order having been made.
3. The application for leave to appeal out of time should be made reasonably promptly in the circumstances of the case.
4. The grant of leave … should not prejudice third parties who have acquired in good faith and for valuable consideration, interests in property that is the subject matter of the relevant order.
Barder itself graphically illustrates circumstances in which leave might be granted. In that case, by consent the husband was ordered to transfer his interest in the matrimonial home
to the wife largely because their two children would reside with her. Shortly after the order the wife killed both children, then committed suicide. However, this extremely uncommon event might have suggested that the discretion to grant leave would be exercised only very exceptionally. Since 1988 the courts have shown a willingness to allow appeals out of time in less extreme circumstances. They have allowed them sometimes in relation to the valuations of businesses and, topically, houses but only where the applicant for leave has applied quickly following a material change in value. Nevertheless, mindful of the enormous benefit of finality in litigation the courts have generally discouraged such applications. Recent cases have demonstrated this (Kean v Kean  2FLR 28; P v P (Consent Order: Appeal out of time)  1 FLR 743).
Please supply more details of the facts behind the order and how it was made
Obvious solution is to suggest her fixed sum is reduced by a % new value has against old value.
Court of Appeal has warned solrs the dangers of agreeing fixed sums and a pity this was the case -were you advised of the risks??? Possible negligence issue
Hope this helps