Jipper, the term used doesn't matter: you can call it by what term you believe to be current in English; the point is that an au pair is not a servant, a domestic worker, a maid, a nanny, or any other kind of employee or 'worker'. She is an au pair, don't you understand that, and is to be treated as an au pair?
If you want a live-in nanny, an employee, that is, not an au pair, expect to have to provide her with her own separate accommodation, such as a staff flat within the home,and pay upwards of £600 a week, all found, net of all taxes. You may be lucky at that; foreign families often pay a £1,000 a week for British nannies to work abroad.