Charities proving prior contact

Good morning

With regard to GDPR, charities tend to delete old, lapsed records of supporters after a certain number of years which can be problematic when needing to prove prior contact for legacies.

Can anyone advise how much information is needed to be able to meet a satisfactory level of proof, is just having the details of the person on record enough, or does it also need to show the types of contact (such as donations etc). If so, is it good enough to show, say a total, or should each point of contact be retained?

Many thanks