i think you may be new to themanagment role. As a contributor has said training is the first step. and much beetter if everyone has the same training so that you cannot be aqccused of victimisation.
Then you have ACAS behind you. This is the leaflet on how to manage staff
www.acas.org.uk/media/pdf/p/0/How-to-manage-performance-advisory-booklet.pdf
They have a phone number they can help you out with information. You will need to go down the capability route.
Also has you firm got an HR department? Then they will help you.
Always record every formal meeting and every informal chat.
As part of the capability you might be able to put her along side someon who performs well to show her how things sould be done.
Also can this person see appropriately. I have a cleaner who has poor contrast sensitivity and wears reading glasses. So half the time she cannot see and I have to point out cobwebs etc.
Sadly my husband gave her a rise.
Baby Reindeer - anyone watched it?
Our Welfare State. Is it broken?