Ask for a Third Party Mandate form. This gives you full access to her account including a card and check book. Really useful if you are doing a move, and is effectively a way of bridging the gap between taking over finances and having the POA come through. . Or have her request internet banking and put it on your PC. (Also set up an email account for her which you access. Over time you will find that you need to do more for her and it is so much easier doing it all from home.)
The earlier you take over the admin side, the easier it will be. My dad had everything neatly organised, and then my mum was on her own refusing help for three years. I ended up trying to sort the mess above, over 50 bank accounts (I kid you not - some were stock market linked with very very restrictive withdrawal policies - sold to a vulnerable person over the age of 80!), three years of missing tax returns, utility bills that had been changed every month each time a new salesman phoned, and an Inland Revenue enquiry, and the remains of my dad's probate. It took over a year.
Four years later I have just about finished updating my dad's rental properties so I can attract sensible tenants, and perhaps can sell them.
I feel very tired of it all. So heartfelt advice. Calmly announce you need her to complete the third party mandate so you can carry out internet banking from home. Nominate someone in the family who is willing to do this. Then sort out direct debits etc so workload is minimal, and have day to day expenditure in a separate, non-overdraft account, so any losses/scams are kept to minimum, without her realising that she essentially has lost control.
I have got better at an assertive approach which avoids discussion and argument. Fibs, though, are essential to provide a way for her to agree.
One thing that helped my mum (I was really shocked and worried about the extent to which she gave out bank details over the phone, so had to act quite abruptly) was providing her with a written summary of her financial affairs. She still worries a lot about "not having any money". So access to money when she needs it, and confirmation that there is enough to see her through, made a real difference.