No licence is needed for events which are private functions, or invitation only events.
The only time a licence is required, is for open air events (Carnivals, Festivals, Sports Events etc) and for any event where a fee is charged or tickets sold. Pubs and Nightclubs also fall under this banner. Rumour has it that Office parties, require a licence too, although I don't know why this wouldn't be covered as a private party - especially if it was held at a local pub or hotel function room

. It is a private party under these circumstances, in my opinion.