Holiday entitlement looks wrong.
Holiday Calculator - I get 16.8 days (which can include Bank holidays). So rounding up, 17 days. So if you give 12 days, then you have to have 5 working days every year falling on a working day to make it up to 17 days.
A Monday is NOT in the working days you propose, so I don't think you will get 5 bank/public holidays falling on the working days (tue, thur, fri).
Have you checked against a calendar to confirm what the actual number of days off would be say during 2012, if they were to be working Jan-Jan (I know you are doing May-May)?
.25 hours, hmm... have you allocated some time for handover? Is having some additional overlap worth while, say for occasions when you might be a little late, or is that already accounted for in the hours?
I make salary roughly £8.99 an hour Gross. Not sure how that compares to nanny jobs in Chester - advertise the job and see what you get.
There is a potential issue... you would like someone Ofsted registered but you would also like some with a recent CRB check. If someone is Ofsted registered, their CRB check (candidate copy) could be several years old as it is done at the time of initial registration. Ofsted say they then get updates... but the candidate never gets their paper copy updated.
If you want someone to have a recent CRB check and they don't have one which is recent enough in your view... how will you get a recent one - parents can't apply for one and neither can nannies (unless they go via a nanny agency). I am guessing you don't want to pay agency fees... so how will you get an up-to-date CRB check?
Trial period - is 3 weeks enough?
Babysitting - no mention about payment.
You may want to mention what meals for nanny are included, given the working hours I expect you would want nanny to eat with the children for Lunch and Tea.