Bless, that is amazing.
Could not the vet be the proof of age, as Bella is still alive her records will still be current, surely Bella must be on record for her yearly booster, from when they first had her and the approx age at the first session would still be written down on her vet records, that should be traceable.
Even if now at a different vet would her records not have been transfered, I know when I changed vet they were.
The RSPCA are surely not the only proof.