If they are working 5 days a week for you, would they then also want to come back at the weekend? On occasion perhaps they would but having that in the contract is I feel expecting too much.
Salaries will vary I feel. As the employer I feel you should be deciding how much to pay, then advertising the job at that salary and seeing who applies. You seem to be doing it the other way around... advertising the job but without a salary, then asking people what they want to be paid. Personally I feel that wastes peoples time, as you may love a particular candidate but find they want a salary you can't afford to pay.
Using MrAnchovy's PAYE Calculator 450net for 55 hours per week, assuming 810L taxcode and no student loan deductions or any other strange things, then it's around £30,780 gross a year. Certainly not a bad salary, but do look at the cost of places to live in your area - a live out nanny will be living quite close I would suspect, so are house/flat prices/rents affordable if the nanny is on a £30k salary?
It is easier to increase salary over time, then to decrease it. So you can reward someone you feel is a great at their job by giving them a salary increase... perhaps following completing probation, following completing 1 year of service, that sort of thing.
What you don't want to do is offer a salary which you can't afford to pay and can't afford to raise over the years.
I feel there are lots of people looking for jobs and not that many jobs, so at a salary of say £11 gross an hour, I suspect you should find quite a lot of candidates. It's certainly more than I get, but I'm 22 miles from London.