Can our annual internal audits be conducted by an external auditor (say…yourself), or must they truly be conducted internally by a member of staff not directly responsible for certification?

It can’t be conducted by me… I’d be auditing much of my own work (a no-no).  I can recommend several good external auditors, or you can have external staff conduct the audit.  The problem with external staff is that 1) they typically don’t know much about certification and 2) they aren’t familiar with the 17024:12 standard.