Liability insurance is paid yearly (current cost £60 if the nanny is registering or is already registered with Ofsted)... not sure it can be done monthly... it may be possible but it may be like other insurance whereby it's done via a credit agreement. Your nanny would need to ask the insurance broker.
As part of the Ofsted registration process, Ofsted will do another CRB check - it is that check which can take time, so registration could take a few weeks or longer if the nanny has had multiple addresses over the past 5 years.
excuse my ignorance but what are the main differences between a childminder and a nanny?
A childminder works in their own home and cares for children from multiple families. A childminder runs their own business. You use their service, you can not tell them what to do. Childminders caring for children for 2 hours or more in any day need to be registered.
A home childcarer (nanny) works in the home of the family and cares for children from just that family, or sometimes from 2 families (a nannyshare). A nanny is an employee, they work for you and you can tell them what to do. Nannies do not need to be registered, though can be registered as a "home child carer" so that parents can use Childcare Vouchers or Tax Credits.
I assume if she calls herself a nanny I will be her employer where as if she is a childminder she is self employed?
Employment status does not depend on the job title... it is the nature of the work which is one of many factors that determines employment status.
If they come to your home and care just for your children, then they will be your employee. You will need to operate payroll - you can subcontract that out to a payroll provider such as www.payefornannies.co.uk who will help you register as an employer, help prepare a contract of employment, provide monthly payslips, remind you when to pay money to HMRC, that sort of thing.
MrAnchovy's PAYE Calculator can help give you an idea of the financial figures involved. As the employer, you pay your nanny a Gross Salary. You pay Employers National Insurance to the Government on top of that Gross Salary. From the Gross salary you deduct Employee Income Tax and Employee National Insurance, which results in the Net Salary figure (the amount which you pay the nanny as their take home pay). You pay the employee income tax and employee national insurance you deducted from your nannies salary to the Government. Sounds complex but it isn't once you get used to it and a payroll company will make it easy for you - the payroll companies charge around £130 a year.
For tax credits purposes when making your tax credits claim you are permitted to include in the cost of childcare: nannies gross salary, employers NI, payroll admin charge. Think it's WTC5 which says about that, though it may be WTC2.