You can object to the registration of the Lasting Power of Attorney if you can prove that your mother didn’t have the mental capacity to enter into the arrangement. The procedure is shown here, along with the application form for the objection. You need to read all the pages because it’s a different form under different procedure depending on your position in this. If in doubt, get a solicitor to make the application for you.
https://www.gov.uk/object-registration
you can also apply for your mother to make a Statutory Will or rather to have one made on her behalf you would only do that if the rules of intestacy (having no will) were going to be less favourable usually from a tax point of view.
It is necessary to make a court application and here is a little reading on that.
http://www.freethsoxford.co.uk/applying_for_a_statutory_will
Please note that it’s only on rare occasions that it’s worthwhile making statutory will.
Can I clarify anything else for you?
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FES.