Access denied when disabling agent job, despite SqlAgentOperator membership
Please check to see if there is a DENY on the sp_update_job procedure.  This may be a direct DENY to your login or it may be a DENY on some group that has your login as a member.  DENY  always trumps a grant.

If so, then removing your login from the group or having an administrator REVOKE the DENY should solve the problem.

Response prompted by reading Piers7 request.  Time has passed, of course.