I've started to populate the suppliers module but I have a real issue with "Account Manager" which is a drop down list of people (which is naturally my own staff database). Why should I have to record firms' account managers in my database to allow this selection?

This would appear to me to be a software error.   

This is used normally to provide a point of contact with the supplier.  So for example, you could have Gael Ltd listed as a supplier for Q-Pulse and you could list your Gael account manager in this field so if there are any enquiries you can direct them to the appropriate person.  This field will display employees and contacts - the account manager will be entered as a contact and not an employee.