Coining it

I am confused about the operation of gift aid. I have some elderly clients who tell me that they often drop a few coins into collection tins as well as making some more substantial donations.

On some occasions these larger donations have been accompanied by gift aid certificates that claim tax relief on all donations made in the last six years to the same charity.

Am I supposed to rework the tax relief for cash donations when my client has no real idea how much might have been given and when? If so how?

Does the obligation change if for some of the years my clients might be repayment cases – any repayment being limited by reference to the tax due on the charitable gift?

Similarly how is a charity supposed to be able to...