Call from her bank to my very elderly mum.

I can't imagine any bank phoning your mum out of the blue.

Contact your mums bank.

Buy her a phone which only allows people from her contacts list getting through.
She only will need to put the phone down they will soon get tired of ringing if it's a scam.

She will also be able to block numbers.

I support an elderly ( can get confused) neighbour who gets regular phone calls from NatWest. In fact last week they sent her a beautiful hamper.
I too am a little concerned that she might fall for the lovely young man on the phone
I do, in a subtle way try to educate her in these calls. I have to be careful as she gets very defensive if there’s any suggestion that she might be getting confused.

I would go into the Bank, preferably with your mother, and ask them about these calls, who is making them and why.
It does sound like a scam to me.
If it is a genuine concern to protect an elderly customer, you can set up a code/password for future contact.

Scam or not, it means your mother is more like to respond to a spam call when she does get it because she is so accustomed to getting regular calls from the bank.

The bank should be told, in wrting, with your mothesigned consent that if they need to phone her they must phone you, possibly change the phone number on the account to yours.

It then means that your mother will know with absolute confidence that anyone ringing her claiming to be the bank, are scammers and she should just put the receiver down.

I would be concerned for the same reasons as you are nonogran
TBH I dont know if it is a scam or not but it certainly sounds as like it.

I’d definitely get in touch with the bank. I’m pretty certain they’ll confirm this is a scam. Your mum’s being groomed.