Nannies Duties - be as specific as you can about the major things you want doing. Then have the last clause in that section such that it enables you as the employer to add to the section - such as, "any other duties required by employer and agreed by nanny" or some such thing (I'm not a lawyer, so not sure of correct legal wording, another mumsnetter may know).
Think carefully about the duties - especially if you are expecting nanny to do children's washing, any housework. "Nursery Duties" does not in my view cover it... you need to spell out exactly what you require - then it is much harder for there to be a dispute later on.
Be careful using a template contract... some of them in my view are not that great. For example, the NannyJob Specimen Contract says:
"1.6 Unless prevented by illness or injury the Nanny
(c) not perform any paid or unpaid work for any third party without the prior written consent of the Employer."
My view is that what the nanny does outside of their working hours, is up to them. I don't feel it is reasonable for a nanny to have to obtain written consent from their employer, every time they babysit for a family down the road! What the nanny does during contracted hours is something you control... what they do outside of contracted hours I feel you are not able to control.
Be very clear about holiday entitlement, notice periods required to book holiday.
Sick Leave... try to have it as SSP straight off. Then it is up to you if you pay for the first x number of days sick, or not. If you put in a contract that you pay for x number of days sick - then expect the nanny to be sick that number of days per year, regardless of if they are really sick or not.
Gross Misconduct - I would suggest listing as many examples as you can think of. This then makes it very clear what things are considered Gross Misconduct, and things not included are then treated under Disciplinary Procedure.
If you require your nanny is registered with Ofsted/Childcare Approval Scheme, than add that requirement to the contract - also detail who is responsible for associated fees.