Looking on Admiral website one, but not the only, reason for a £26 admin fee is for applying additional information.
Perhaps there was some discrepancy on what was submitted at the time of claim and what was submitted at the time of policy.
Misrepresentation or change are the 2 reasons the policy allows premium change (and the 2 reasons for a £26 admin charge).
I assume the incident occured after the new policy was quoted and commenced.