when you employ a NANNY you have to be an employer...
you are therefor responsible for their tax NI, and employers contributions as well as their net (take home) pay.
There are a few companies (such as nannypaye ) who will take care of all these things for you. they will also advise you further on the ins and outs etc!
Basically as an employer you are responsible for all 'normal' employer things, such as providing pay slips, and if you make her redundant (ie dont need her anymore after shes been with you for 2 years or more) pay redundancy pay.
The nanny will also have the same maternity and sick rights as any other employee in any other job, that you will also be legally obliged to honour (tho im sure ive herd that the government reimburses you for maternity pay / statatory sick pay etc)
a nanny like any other employee is allowed the statatory minimum of 4 weeks holiday per year(which can include bank holidays) like any other employee.
The convention is to make it 4 weeks plus bank holidays (tho you arnt legally obliged to do this i dont many of us who would be happy to sign a contract if those 20 days included bank hols!) also generally the nanny chooses 10 (or half) days and the employer 10 days. (again the employer can choose the full 4 weeks, tho i cant imagine many nannies being happy to have that in their contract either!)
you also need to insure you have adequate household insurance (which i think isnt much extra (if any) as long as you notify your home insurance provider)
hope that helps!