A bank or building society will accept instructions about an account from a specific person named in a third party mandate signed by the account holder. The mandate gives that person the authority to run the bank account for the account holder. There are usually some restrictions on what the third party can do, such as not being allowed to arrange a formal overdraft or open or close an account for the account holder. 

However, a third party mandate is not appropriate if the account holder is losing the ability to make the relevant decisions, as it will be invalid once the person has lost capacity.   

Therefore a registered Lasting Power of Attorney, a registered Enduring Power of Attorney or a Deputy Order will be required for ongoing management of the account and other assets.  Sadly it is reported that there may still be hurdles for the authorised person even after the legal paperwork has been put in place.