Please specify a gross salary or hourly rate. There are various different issues with net pay agreements relating to the Nannyâ??s tax code and circumstances and so often parents aren't aware of them until it's too late and they are having to pay lots more for their nanny than they were expecting. 
The main issues are:
Outstanding tax owing: The Employer has agreed to pay the Nanny a set amount each payday. The employer is therefore liable for any underpaid tax the nanny has incurred during the current tax year outside of their employment (or indeed for a previous tax year if underpayments are to be made from lowering the nannyâ??s taxable allowance so that s/he effectively pays more tax).
Tax refunds: On a net pay arrangement, the total amount of tax being paid over to HMRC is still being deducted from the Employeeâ??s True Gross pay. Often, in instances where tax refunds are generated the Employer sees the money as owing back to them but technically the tax rebate should be refunded to the Nanny, not the Employer. This can cause major problems between the Nanny and employer.
Tax code errors and amendments: Say a Nanny brings in a P45 from her/his previous Employer with â??BRâ?? or a low tax code (for example if she previously had 2 jobs.) Unless the full tax allowance is issued the PAYE payments will be calculated using higher amounts of tax (which in turn creates higher levels of National Insurance contributions due too). Any over paid tax should be refunded to the Nanny but any overpaid National Insurance can not be claimed back.
There can also be issues where tax codes are on a month 1 basis (where previous earnings and tax that has already been paid is not taken into account). Again, any overpaid tax should be paid back to the nanny but any National Insurance that has been calculated may have been much lower had a cumulative code been in place in previous calculations. This National Insurance can not be claimed back.
The Employer could also be liable to pay student loans, court orders etc on behalf of the nanny. Net pay arrangements just aren't worth it!
Have a play with Mr. Anchovy's calculator: www.mranchovy.com/calc/paye201415 and work out what you can afford but do bare in mind your Nanny might not have the "normal" tax code of 1000L which the calculations are based on.
Hope that helps! 