taking on my childminder as a nanny

[teabagtea]

I use a childminder one day a week. She has been winding down for a while and now most of the older children she minded are at school and my DD2 is the only one with her. She doesn't want to deal with Ofsted etc anymore for just one child, and would like instead to nanny for me for one day a week. I have 2 other DS's that I could ask her to pick up from school instead of going to after school club. BUT I've never used a nanny before--- can everyone PLEASE let me know what issues I have to deal with / think about before going ahead with this.. I don't even know how much nannies are paid ( I'm in |Buckinghamshire). Thankyou!!

Nannies pay varies, as the employer I feel it is you who decides how much to pay. For a live-out nanny who is working outside of London, £7-£11 Gross per hour is probably about typical. The more experience they have will reflect the higher salary.
You are their employer, so you need to deduct their Tax and NI from their salary, plus pay Employers NI. As it is one day per week, you will probably be able to register as an employer under the Simplified PAYE system (max weekly earning of £160).
If you wish to part-pay via Childcare Vouchers, then your nanny would need to register with Ofsted (it is a different part of the register that childminders register on - not quite so demanding, though Ofsted can still inspect a nanny).
Consider hidden costs - heating/light/food... as care will be at your home, your household expenses will increase a bit. Nannies also get food provided whilst on duty - typically meaning Lunch plus drinks and could include Tea/Dinner if working late.
Also activities your DD2 attends would be extra cost... nannies often get weekly kitty money to cover outings/misc expenses.
They are an employee, so are entitled to all the usual employee benefits - Paid Holiday, Maternity Leave etc.
Also consider use of a car. If the nanny provides their own car, they need suitable insurance cover plus you as the employer will probably be expected to pay 40p per mile (this is the HMRC approved rate), mileage allowance.