Can we add up days over year, then divide by 52 so I get a weekly amount regardless of how many hours I do?
Yes, but it can create problems when it comes to ending the agreement. So it is vital that the actual hours worked each day are logged, so should the job end part way though a year, the correct amount final payment can be calculated (that payment may actually be a debt - you may owe them money).
Why weekly pay? Would you accept monthly?
It will come to about £160 a week so no tax? NI should be a couple of £
a week. I have advised them of Nannytax company. Is this quite easy?
Will this be your ONLY income? No money coming from anywhere else?
The monthly NI Lower Earnings Limit for 2014/15 is £481 per month.
May be best to use the monthly figure. If you are paid more than that then National Insurance will start to apply, also Employers NI will start to apply depending on salary. Payroll is easier to do monthly than weekly in my view. If using a payroll company, then they will often charge extra to do weekly payslips.
If not using a payroll company, then they can use the HMRC online system and handwrite payslips, or use lowcost software (think it's actually free for first 3 employees) BrightPay.
If using a payroll company, suggest they shop around for one which is lowest cost for what they want to do, as the payroll should be quite easy. The difference in cost will probably be down to if monthly or weekly payslips are needed. They should also consider if they need access to support lines, such as employment law, which some payroll providers include in their fee. PAYEforNannies often gets mentioned on here, currently they charge £125 for monthly payroll, £230 for weekly.
MB will be v busy and I don't want to add to her stress.
Suggest she calls PAYEforNannies or NannyTax and gets them to set everything up for her. First though, decide if you really need weekly pay, or if monthly will work. Also work out the annual hours.