Have you established the salary you are prepared to pay and are the candidates meeting your person specification?
Depending on how much information was in your advert - have you been in touch with the candidates to give them a full job description and relevant background information about the way your family works, the things the children like doing, that sort of thing?
Have the candidates submitted a CV for which the basic details check out - date gaps explained, names of schools and colleges valid, their nationality is such that they don't require any visa/work permit or things like that.
Arrange to meet them, so you can have a chat about the job in person and have a look through all their documentation. If you hold that at your home, say in the evening, then the candidate also gets the chance to show they can find your home and arrive at a specified time. Take into account what hours/days they are currently working - find a suitable evening and time that suits you both.
The one you gel with best, invite back for a daytime meet with your children present. See how they get along with your children.
When I started my current job... it was a July start, by this point I had accepted the job. So a 4 month lead time. So for you to interview now for a September start is not too early in my view... you may well have it all sorted out come early May.
I'm in West Surrey, near the Berkshire/Hampshire border. If you already have candidates for the job, you don't need to use a nanny agency. Agencies I find can be great at times and hopeless at times. Based on agencies who have found me a job in the past... thus are a good agency from my point of view... try Hatties in Godalming - though depending where in Surrey you live, they may not cover your area.
If you want an opinion on suitability of candidates based on their CV, do get in touch via e-mail or PM.
Also I would like to use an agency to help with wages and contracts, is there an agency that do that even if they don't find the nanny for you?
PAYEforNannieshas been doing my payslips for over 3 years... I am happy with what I get from my boss... she seems happy with them. So they are worth a look. If you prefer dealing with someone local - then depending where in Surrey you are located, I know of a payroll company based in Surrey. Though really a payroll companies location makes little difference, as you deal with them mostly by phone and email.