a) Can I hire someone to nanny during school term only from eg. 2-7.30pm then holidays from eg. 8-7?
Yes. You're more likely to up your chances if you have a nanny with their own child as they won't mind mornings free in term time.
b) How much do you pay a nanny? (we live in essex, near london).
£10gross is a reasonable estimate (plus the employer's NI on top). A nanny with own child may accept £8 or 9.
c) Can you pay a nanny a fixed monthly salary to cover short hours in term time, then longer hours in holidays?
Yes as long as you're aware it will make things complicated if they leave half way through the year.
d) Could I hire an au pair? are they qualified to look after 3 year olds? How much would they cost?
You could, depending on the au pair. Do you have space for someone to live with you and are you happy with that?
e) What websites can I advertise my job on?
In addition to the above gumtree, MN local, nannyjob.co.uk
f) What rates do nannies charge?
It's not about what they charge, it's about the salary you offer. See above for what would be reasonable.
g) How long should I leave myself to find someone?
The market is quite good at the moment but I'd still leave 6 weeks to allow for someone to work a notice period.
www.bapn.org.uk should also have some guidance which would be helpful.