The nanny wants £11 take home pay per hour, working 20 hours per week... is that right? So £220 net per week.
Do you know what their tax code is... probably not. It could also change at any point. So how can you do Net pay?
You can use a payroll company and tell them that you want to pay someone Net and get them to work out the Gross figure each and every payroll run. The nanny payroll companies have computer software that can do that. It is not something you could do yourself very easily.
You nanny could have a BR tax code for this particular job. This would be due to them using all their personal tax allowance in another job - which may well be the case if they are only working 2 days a week for you... there are another 5 days they could be working for someone else.
So you could work out the worst case situation and thus know roughly how much to budget for in terms of payroll cost.
Does the nanny have an outstanding student loan - ask that question, as if they do repayments for that are taken off at source, so DON'T agree a Net wage.
Using www.mranchovy.com/calc/ you can get some figures for your likely costs. Note that the figures may be a little different when it comes to doing the payroll run and will certainly be different come next financial year (so from April 2012).
So figures I am entering into the PAYE calculator:
11 Net per hour, 2 days per week, 20 hours per week, tax code BR, student loan NO.
Result as Yearly figures:
Gross Salary £15,595
Income Tax £3119
National Insurance £1004
Net Pay £11471
Employers NI £1176
Cost to Employer £16771
Do you see how the cost to the employer figure is calculated (Gross Salary + Employers NI). You will then have other costs to add on top, including Payroll Admin charge (typically around £135 a year).
You pay her the Gross Salary, from which you deduct Income Tax and National Insurance (you put those aside in a savings account). Also add the Employers NI for that month to the savings account.
You give your nanny a payslip showing the deductions made and pay them the NET pay - the figure which is left after the deductions from the Gross salary (Gross - (Income Tax + National Insurance))
Every 3 months (per quarter) you send HMRC the Income Tax, National Insurance and Employers NI. The payroll company will tell you when to do that, how and how much money to transfer.
If you were to agree a Gross salary you could do the payroll yourself. However even then you may prefer to pay the admin charge to have someone else do it for the first year whilst you get to learn how it's done.