It depends on what you are looking for and where you are. I'm an after-school nanny, in Ealing, which may be too far for you. It is also important to make sure that anyone you are selecting, knows what kind of care you expect in school holidays. This can be full-time, or if the children go to playschemes, it could be the same sort of hours.
In my previous job, I did before and after school care term time and then a few extra hours in the hols, to take them to playscheme and collect them again. I also got paid for the holidays when they were away. If you don't need your nanny during holidays, you might find you have to pay a retainer, usually about 50%.
You would also still be liable for their tax and NI unless they were self-employed. I am self-employed as when I started this line of work, I also had a daily maternity position.
You could perhaps consider a share arrangement for after-school, or an arrangement with someone who does mornings in another job, or works a few hours during the school-time. If your employer allows childcare costs, it would be worth getting an Ofsted registered nanny, as they will allow you to use childcare vouchers and will meet certain standards in terms of qualifications, insurance, crb, first aid etc.
Two good sources of information you could try are:
www.simplychildcare.com [they have a very good childcare information pack available] and www.afterschoolnannies.com
HTH