How much to pay a temp nanny in Wandsworth

Wonderful Gurus of MN.

Help!We are about to hand in notice to my CM as i just can't trust her anymore. Long back story but involves her dog scratching my sons face, yes an accident but i have yet to see the accident log, poor communication, poor nutrition and lack of daily trips out, her ratios are 1 of her to 2 under ones and then my son at 13 months and a 2 year old. I think this is against some rules, but she dismissed me when i pointed this. Maybe my expectations of a CM are too high? Notice being handed in tonight, 2 whole months of bloody notice for inadequacy ... hey ho. Can you feel my stressing? I am literally losing sleep over this now as my options are limited.

So he is hopefully going to get a place elsewhere maybe in Feb 2017 depending on movement. So i need a nanny for Jan - Feb but poss longer. There are no other options near me as all full, longer wait lists etc and i cannot let him stay longer somewhere i feel the trust has gone. We can , we think, just about afford a nanny for 2 months or so but not longer term.

My questions - this is a a temp position, 745- 6.30 4 days a week.

1) How much do i pay and how? Weekly? Hourly? Monthly? Gross?
2) What sort of contract is this? Is there such a thing as temp , fixed temp contact for a Nanny?
3) she adhocs for an agency for travel nannying at weekends. Does this matter?
4) She put it on me to offer a salary? We pay £12ph cash for adhoc nannying ( ie when son is ill and i need to work and last minute texting the night before if she free and bank transfers) She is very sweet but a bit clueless financially.
5) Is it too complicated to do the PAYE myself? Am trying to be upright here and do it properly, to protect both her and us.

So... MN lovelies, how should i proceed? How much to pay and how to negotiate a contract?