Hiya, with 3 DC, you may find a nanny is definitely your best bet. Your list of requirements is certainly normal - and the 'early' finish may well be attractive to some.
I would say you could try looking at around £10 per hour gross, you may find someone less experienced for that. It's worth a look. You basically advertise what you're prepared / able to pay. That would bring your salary costs to £90 per day (gross). We are in the South East and pay our nanny around £10.50 per hour, but a couple of nannies we interviewed did have lower expectations.
If you have an Ofsted registered nanny, you could use Childcare vouchers toward her pay - that would save some money.
I have my nanny in the hols as well, so can't say what my experience of term time only is... for some I guess this would be a problem, but again, you don't know till you try.
If you choose a nanny who brings her own young child with her, that may work in your favour, in that
a: you can negotiate a lower hourly rate
b: they may only be looking for part time
I would advertise - www.childcare.co.uk and nannyjobs are good starts - put up your requirements, put up your pay scale, and see what the interest is.
Although you definitely have to factor in all the costs, I do think you can scare yourself off before you start if you account for every single penny - after all, you have to pay your own petrol if you are doing all the to-ing and fro-ing yourself, and feed yourself etc, so the extra you have to pay a nanny may not make 'much' difference to the bottom line in the long run.
Good luck.