It's a bit more complex now RachieB. There are different types of childcare provider and different time periods for those differing types of provision.
For anyone considering caring for children in England, the following are useful documents to read:
Registering Childcare Providers from September 2008
The Childcare (Exemptions from Registration) Order 2008
Temporary provision is exempt if it is for two hours or less per day, or for four hours or less per day and the provision is offered on a day to day basis with no longer term commitment to clients, and for the convenience of clients who intend to remain on the premises where the provision is made or within their immediate locality.
There is also a 14 day grace period, so if the care is not provided at the same location on more than 14 days per year, then it is also exempt, subject to Ofsted being notified at least 14 days in advance.
In the case of Childcare provided within someone's home... then I feel it is reasonable to say that if such care is provided for more than 2 hours per day, it isn't exempt (as I feel the 4 hour rule is more for a crèche, where the parents are on the premises or immediate locality).