I may get shouted down here, but although 'officially' an aupair is only expected to work 25-35 hours a week, and in theory is here on a cultural exchange programme, there are a lot of people happy to take an aupair-type role, (about £150 pocket money/week plus board etc) and do a lot more hours.
A lot of these are girls/women with substantial childcare experience in their own countries, where they either can't get jobs due to the recession or who want to improve their long-term career opportunities by improving their English.
Obviously you would need to be very selective, and careful with references, if you went down this route, but I have recently been advertising for a new aupair and was inundated with applications from Spanish, Italian, Hungarian, Latvian etc girls who had heaps of experience but wanted to come to the UK for a bit.
Quite a few of the mums I know have had long-term aupairs on this basis.
If you are interested, take a look at aupairworld dot net to get a better idea.
I will caveat all of this by saying that I had a full time live-out nanny at the time dc1 was that age....now I have 3 dcs in full time nursery/pre-school/school we have a 'normal' aupair to help with mornings and evenings, and that works brilliantly.