A live-in nanny on £400 per 5 day week generally has childcare qualifications and experience. My first job I got £30 a week! I only did 2 days, one week 7am-7pm the following week 7pm-7am, it was 30 years ago!
I would get her to do a paediatric first aid course so she knows what to do if anything happens. They cost around £80 which you should pay for. I would also do a long hand over, at least 2 weeks but more if possible.
You need to talk to her to see what she is expecting but I would say £180, that's £6 per hour and a pretty good wage for an 18 year old with no formal experience, you can count a babysitting in that but if you are late home you should pay her overtime or count that as your babysit.
However, you have to be aware of all the extra factors of having a live-in nanny.
Wage - you will have to employ her legally, you will have to pay her tax and national insurance and employers national insurance as well. If you can't do it yourself, you will have to pay for a company to do the tax for you, around £200 per year. On £180 it comes out to £9,805 a year. £150 is £7,821.
Heating - if she's home and a chilly mortal, the heating could be on for 30 hours a week more than if your child was at nursery.
Food - 3 meals and snacks which can come to £30-40 a week. I would put in the contract and tell her that you will pay for the basics, cereal for breakfast, sandwich or soup for lunch and main meal with you and a few snacks (you have to provide food 7 days week even though they only work 3. Clear out a shelf for her snacks/juice and tell her this is what her snacks are and once they are gone she needs to replenish herself, there a lot of threads on here about au-pairs/nannies eating the employers out of house and home so easier to have the ground rules in place from the start.
Kitty - taking your child to toddler groups, music class, gym, swimming, outings. In London these classes are pretty expensive, my boss pays £23 a week for my charges classes and swimming. Most nannies also use kitty for picking up bread, milk, dry cleaning, the occasion coffee/tea/cake even lunch out (most of us will take picnics on outings but sometimes you can't.)
Travel - my boss tops up my oyster with £20 once a month to take my charge to various places.
Gifts - you probably should buy her birthday and Christmas gifts, maybe also the occasional thank you gift. For my first Christmas here, I got a Kitchen Aid!! You obviously don't have to be quite so generous! My boss occionally buys me flowers, plants, chocolate to show her appreciation, which is really nice.
Wear and tear - it depends how clumsy your nanny is but there will be breakages. I crashed 3 cars in one job! (Only one was my fault, the other two, I wasn't even in the car but the insurance went knock for knock on both of them, my employers would not let me pay the excess on any of them and they still let me drive their cars when I look after the boys at weekends/holidays now!!) In this job, I have smashed 1 bowl, 1 plate (both Emma Bridgewater £15 each) a few glasses and the lemon juicer (but that already had a crack in it and it broke when I put it down on the work surface) all in the last 6 weeks! I have been here 2.5 years so not too bad.
That's a lot to think about! Good luck. I hope it works out in the end.