Salary sounds fine, not that I know where Zone6 is but I suspect on far outer London. A qualified, experienced nanny may well get/want higher but this person is not experienced or qualified, so you could well offer less.
Check, check, and triple check your budget. You are doing the right thing asking on here about what costs are involved. Look through past message threads, there are some which give some comprehensive cost calculations, though they may be a few years old, but will show things to think about.
I would factor in the cost of a payroll company, so you don't need to get too involved with payroll admin. Cost is 100-250 a year, depending on what you need. Monthly payroll is cheapest.
Employers NI varies with salary. www.mranchovy.com/calc will give Employers NI figure for 2013/14 tax year. Not sure it does 2014/15 yet. Not sure if Employers NI is changing from April.
Consider the small costs... Food/drink for nanny whilst on duty, travel whilst on duty, activities, craft supplies. Things will vary over time as children get older.
If working same number of hours each day, holiday entitlement can be calculated in Days. Otherwise calculate in Hours.
Minimum entitlement is 5.6 weeks x hours or days worked.
So if 3 day: 5.6 x 3 = 16.8 days. You can round UP, you can't round Down. So 17 days would be reasonable.
If a bank holiday falls on a working day, you can insist that it is taken as annual leave.