Freelance administrators, office managers for hire, PA contractors and admin-services businesses occupy the modest end of professional risk – but "modest" and "zero" are different words when your work includes other people's diaries, payments, filings and inboxes.
Buy the small policy the trade needs – £1m PI now costs pennies more than £100k – and put your seriousness where the risk actually lives: payment-handling declared, cyber cover on, two-channel verification for any bank-detail change. Admin insurance is cheap because admins rarely cause disasters; the exception is the inbox, so armour that.