Hi - we just found a nanny by posting on Gumtree - in my experience you will not find a nanny throught the SE1 web site, unless you are very lucky.
Also there is no such thing as an 'ofsted registered nanny' - ofsted only register childminders in their own home, not nannies operating in your home.
You can CRB check a nanny or find on with a valid CRB, but a new check take 2-3 months so is largely useless.
We just went on references - spoke to previous families, checked the ID of the nanny etc. and ultimately you have to go on gut feel. Even a CRB check tells you very little - basically only if someone has been prosecuted of a crime, which is very rare.
We pay £10/ hour cash. I hope you find this info useful. PM me if you want more info.
We have an excellent of stead registered key worker from St.Pats Montessouri, she is in between projects at the moment. English, honest and incredible with kids in a firm nursery way. PM me for more details