£450 net seems high. We paid about £360 net in outer London for a live out Nanny, qualified with ten years experience. If you are looking for full time care - these jobs are in short supply at the moment. A lot of nannies are having to settle for two part time jobs or nanny shares as so many people go back part time these days.
Don't underestimate the impact of having someone live in. Even with a live out nanny who we really got on with, it was such a relief to have our home to ourselves again. (And leave it untidy if we wanted to.)
Re nanny vs childminder etc - if you're planning on more children in the future, nanny's work out more cost effective. Also as your child gets older it is easier to incorporate all those after school activities and music groups etc.
re questions - depending on how long you want to keep the nanny, ask about how long they'd envisage staying in one job. Many nannies only like the babies and want to move on when the children get older. It was important to me that the nanny grew with my family (babies, toddlers and school age) as so many friends got through a nanny a year which was disruptive for the children. Ask if they still see previous charges. If a nanny has moved around a lot, ask a lot of questions.
Work out what is important to you and your personal style - e.g. healthy food, lots of outdoor activities etc and try to find a nanny who meets this. Some nannies have very different views on what is healthy eating.
Ask them what they would do if their personal views on something (e.g. how or when to potty train or table manners) differed from their employer's views. Some nannies are very rigid in their views and don't like to change their approach.
Follow up on references with telephone calls. People will often say things in person which they wouldn't put in a reference. Ask 'what was the worst thing about the nanny' as well as 'what was the best'. Ask about timekeeping, sickness records.
Make sure contract includes details of overtime pay, what happens if you are late home, any overnight pay and any arrangements for working away from your home.
Ask about attitudes to tidyness. Whilst a small matter - it is a pain to come home after a busy day and then have to spend part of your valuable time clearing up after the nanny.
Check how flexible they are - what sort of things could they be asked to do which they considered an imposition? One nanny felt that being asked to load the dishwasher with children's tea things was 'housework' which 'the cleaner should do.' One nanny wouldn't pop out for a loaf of bread if we had run out. One nanny didn't feel that cooking and freezing pureed baby food during a babies nap times was an appropriate activity.
See how they interract with the children. However this doesn't always give the right impression. As many nannies feel self conscious until they know you better.