An experienced nanny could well want £12 gross an hour... so £20,649 a year (based on 33 hours per week), which would cost you as employer £22,522 once adding Employers NI. Have a play with PAYE Calculator. Note: rates can change, so Employers NI could well increase come next tax year, April 2012.
Minimum you could pay someone would be NMW, which for someone aged 21+ is £6.08 an hour. If anyone would accept that wage is another matter.
Have a look at jobs in your area... both childcare jobs and non-childcare jobs. What could someone looking for work earn? Looking at childcare jobs, what are typical salaries on offer? It can be hard to find out but some jobs listed on job sites like www.nannyjob.co.uk, www.greatcare.co.uk will give a salary indication, especially Nursery type jobs (listed at Greatcare). For example Level 3 nursery nurse, Chiswick, £14k+ - it does not say what the hours are, I would assume around 40 hours per week.
How about looking at things differently... calculate what you can afford to pay, including as many of the additional costs as you can (such as the activities kitty, estimated weekly mileage, running payroll, Employers NI). From that calculation you can work out what you have available to pay in terms of salary. Drop the amount a bit, so you have some room to give a bonus at end of year, or a payrise. Then consider if the resulting salary is a salary that someone would be interested in having to do the job - then try to recruit someone at that salary. In my view it is much better to be specific in job adverts about the salary on offer and the skills you expect someone to have to be able to do the job. As a nanny I wouldn't look at jobs which don't give a salary, it's wasting my time and the parents time. I need to pay my mortgage, so I need to earn a certain minimum amount.