You seem as though you may be a little confused about the differences between being a childminder and a nanny.
Here is a good site for advice about employment issues. There are people who post on this part of mumsnet who have various amounts of experience being employers, employees, having a background in HR/Recruitment and good knowledge of law. There is also a more dedicated section of Mumsnet... Employment Issues where there are some posters who have a very good knowledge of employment law. Of course any advice you may get on here is not Professional Advice. Other places for employment law advice are ACAS and your Nanny Insurers legal helpline.
I am joining NCMA and hoping to use their contracts (and insurance).
That seems a bit odd. You are not a childminder. You do not prepare a contract to give to parents. As a nanny it is the parents who give you a contract/written statement which details the agreement.
Insurance wise, look at the various nanny insurers and check the fine print of the policies (if the policy booklet is not available from the insurer brokers website then email them and ask for it - they should be able to show you the full document on request so you know what the limitations of the policy are. Many have a summary online such as this from MortonMichel).
Also, how much would you say is average hourly gross wage for nannies in London?
Have a look through some recent message threads as London rates has come up recently. It will vary a bit.
I only plan to work 16-20 hours per week.
I would not have thought that many employers would be that interested in that. I could be wrong, there are all sorts of people out there who want childcare. However jobs of shorter hours like that may just be After-School jobs and even those may want longer hours in school holidays.
2 full days per week jobs can come up from time to time... that may be 16-24 hours a week.
Holidays also cause me some stress. I would like to take about 5-6 weeks per year, and would not mind having that time unpaid.
Perhaps a term time only job would suit you? That though means you would go on holiday during the peek holiday time when prices rise.
How do your families sort out childcare while you go on holiday?
We often arrange things such as the family also go on holiday, or parents stay at home.
Do you agree on holidays at the beginning of employment (as in when exactly the holidays are going to be taken)?
Sometimes that may be appropriate, other times it may be that you need to give a certain amount of advance notice. It really varies a lot between each family and can vary during your time with them, especially as children get older and go to school - thus limiting when the family can take holiday.
And I feel that I should really know this, but do you make observations and keep a file of the children you look after as nannies (as you would as a childminder)
No.
do you have incident/accident books etc?
Yes though I do not detail every scratch, just major things and keep a log of medication given.
What kind of information and filing do you operate as nannies?
Hmm? Such as?
Mileage record is kept, so I can claim mileage payment.