What category should I use when entering an invoice from a casual worker/occasional contractor?

Depends on what they are doing for you. Repairs is self-explanatory and if the contractor is repairing something for you that is what you would use.

Contract Labour is a good catch all for work not specific to a project and would cover temporary office help.

Other than just as payroll how would you classify what work they perform if they were your full-time employees and not temporary contractors.