Just run a job search and turned up some results for nanny jobs in Wales.
Cardiff Bay: £120 net per week, for 15 hours per week. Overtime at £7 net per hour.
Care for 1 baby, part-time, live-out.
Powys, Welshpool: £300-£350 net per week, care of newborn, live-in.
Cowbridge: £300 net per week, 50 hours per week, live-out. Care of 2 children.
Cardiff: £7.50+ per hour (does not say if net or gross), 40 hours, live-out, care of 1 toddler.
From those I would say average pay is £7 net per hour.
Not sure how many hours you would need, shall we say 10 per day, thus 50 per week... thus £7 * 50 = £350 net per week.
Using \link{http://www.listentotaxman.com/ListenToTaxman} £350 net per week for a typical person is around £467.27 gross.
Using E-Gismos calculator , with 467.27 as gross weekly salary, Employers NI is £47.39
If we do it yearly, we could take a Gross Yearly Salary of £24,300, which calculates to around £350 net per week. Employers NI is then £2465.92 per year. Thus it costs you as the employer: £26,765.92 per year to employ a nanny earning £350 net per week.
These figures are purely the salary and employers NI. Other costs will be incurred in employing a nanny, such as weekly spending kitty, providing employees meals while on duty, additional car insurance premium (if providing a car) or mileage payments (if nanny provides their own car).
The above figures should be used as a guide only. While being quite accurate, tax rates change and individual circumstances can affect things. Figures are based on 2006/07 tax year.
Hope that answers your original question regarding the likely cost of employing a nanny in South Wales.