It is generally understood that au pairs are not entitled to the NMW. This is derived from regulation 2(2) of the National Minimum Wage Regulations 1999, which state that under certain conditions, including that "the worker is not a member of [the host] family, but is treated as such, in particular as regards to the provision of accommodation and meals and the sharing of tasks and leisure activities", a domestic worker is not entitled to NMW.
A recent judgement of the Court of Appeal (Nambalat v Tayher and Others [2012] EWCA Civ 1249) included some possibly useful expansion of the meaning of this provision:
"The exclusion is intended to apply to au pairs, generally young women who visit the United Kingdom to improve language skills and who live with a family, are treated as a part of it, and share the family's tasks in exchange. It should not readily be extended to migrant domestic workers the purpose of whose employment is to relieve the family of most of its household tasks. They are particularly vulnerable to exploitation."
The Court also quoted the following statement made by the Employment Tribunal in its ruling on the case:
"We tentatively volunteer a few additional observations of our own. In the first place, it seems to us that the concept of being treated as a member of the family must entail a degree of involvement. The individual must be more than a guest. She (or he, but for brevity we will use the feminine only) must be a member of the household. She must be included to some degree in family life. This involvement must entail taking part, or at least being expected or invited to take part, in tasks and activities which fall outside the scope of the work for which she is employed. It also involves her sharing the family's food and accommodation, which must extend to sleeping accommodation, bathing and washing facilities, kitchen and living space. In our view the notion of being included as a member of a family does not require parity of treatment with family members, but a significant difference in the standard of facilities or services made available to the domestic worker on the one hand and the rest of the household on the other may argue against the conclusion that she is being treated as a family member. On the other hand, we note that the Regulations refer to the provision, not sharing, of meals and we do not read them as requiring that meals be eaten together, or even that the same meals be taken. (But clearly a worker who ate different meals from the family would be less likely to be seen as being treated as part of the family.) The concept of sharing tasks, must, it seems to us, be interpreted in the context of what it is that the worker is employed to do. If a primary employment duty of the worker is to get children ready for school in the morning, the exemption clearly does not depend on that task being shared with other members of the family. To put it another way, the domestic worker does not need to share her role with other members of the family in order to fall within the exemption. It seems to us that the reference to 'tasks' is aimed at routine household jobs and chores outside the scope of the worker's employment which one would expect members of a family to share as a matter of course."
This seems to fit most au pair arrangements fairly well although when I first read "sharing ... bathing and washing facilities" I was a bit worried that this rules out the common luxury of an en suite bathroom for the au pair, but as it also requires "sharing ... sleeping accommodation" such sharing is presumably satisfied by being within the same dwelling as I know a lot of au pairs that use the family bathroom but I don't know any that share a bedroom with a family member!