Employers NI is the other big one, which is calculated based on the salary you are paying your nanny. Use MrAnchovy's PAYE calculator to get a feel for what Employers NI will be.
You should not be paying any insurance policies your nanny needs to work. In the event of a claim if you have paid the premium the insurance underwriter may deem the insurance invalid, as the insurance is to protect the nanny against a claim made by you. Hope that makes sense. Your nanny should consider what level of cover they need... for example, does their current motor insurer provide adequate cover, or will they need to use a specialist broker to arrange cover. Nanny Car Insurance via MortonMichel (insurance broker) - actual company providing insurance will vary... broker will shop around for best price policy based on your nannies circumstances.
You should look into Employers Liability Insurance. This is often provided via your home contents insurance policy. However some insurers have removed this cover to help reduce cost of policies, so dig out your Home Contents policy and read the wording carefully. You can buy it separately but it can be around £150... MortonMichel: Employers Liability Insurance
CRB check is part of the Ofsted Registration. If they are already registered and you don't feel happy to accept the check that Ofsted did at initial registration, then some nanny agencies may process a check on your behalf for a fee... can be quite a high fee - ask local nanny agencies. I would expect it to be £50-£100 sort of range.
First Aid Certificate (every 3 years). Is that a training course you want to pay as the employer, or do you expect your employee to keep upto date with their own training? Courses can cost £100+ can vary by area.
Nanny Payroll service will cost you £120 or so... depending on what you require, who you use. Shop around the various providers to see what services they offer for a price you are prepared to pay. You can do payroll yourself via HMRC Online.
Consider weekly activities budget - this may vary from term-time to school-holiday time. What is a reasonable budget to one person may be different to another. Personally I say that £5 to £8 per day is probably reasonable in most cases. Try keeping a track of how much you spend a week on outings/misc child related costs.
Nannies food & drink whilst on duty. See: Live Out Nannies and their Lunches thread.