Under the rules governing a Power of Attorney - any Attorney (the person who has been given the power to handle the finances) can be called on by the Office of Public Guardianship to prove that they are acting in the best interests of the person concerned.
See here: http://www.publicguardian.gov.uk/suspect-abuse.htm (concerns about an Attorney or Deputy)
Since my sister and I registered Mum's POA we have kept scrupulous records about how her money is/has been spent so that if we were ever called for an audit we have all the details.
However be aware that if the Office of Public Guardianship considers that wrong has been done they will appoint Attorney/Deputy of their own choosing and then there will be fees for this service that have to be paid - there was an article in the papers last year about the £1000's being charged in fees for 'official' Attorneys.