Just checked a random bank (Lloyds) for internet access with third party mandate and found it not allowed, although telephone banking is allowed.
You would need a general power of attorney, both third party access and general power require your mum to have capacity.
https://www.lloydsbank.com/help-gui...ne-to-manage-your-affairs/types-of-access.asp
Rules vary between banks.
@Buskitten
Hi. Nitram, my dad banked with Lloyds and until I needed to get and register a
LPA with them, I had third party access to my dad’s accounts which I managed through online banking and I had a bank debit card for his account. We just went into the bank, filled out the form, I set up passwords etc for the online banking and that was it. I have found Lloyds bank to be very helpful and very Dementia friendly, they also have a dedicated team who you can ring directly, who deal with Power of Attorneys.