No, No, No! That's the Net Pay again"
Kanna, Kanna, can't you see?
Net pay is silly, silly, silly.
Sorry, couldn't resist 
This is mumsnet, we do not talk about writing a blank cheque. Gross Salary is what you need to talk about, so you know what your costs are, not a net salary where your costs are hard if not impossible to calculate. Do not assume that everyone has the same tax code, otherwise you may be in for a big shock if you employ a nanny who for whatever reason (past underpayments, student loan, multiple jobs, to name a few) is not on a single person's tax code.
NannyTree wrote in this message thread (20 Feb 2012) that they placed a nanny in Richmond, 50 hours per week, at £13.50 Gross per hour (£675 gross per week). So about £511 Net if 810L tax code 2012/13 tax year... if you need the comparison though of course your nanny might not be 810L.
I feel it's an employers market at the moment... lots of applicants, not that many jobs. So you can calculate what you want to pay and then advertise the job at that salary. £600 gross per week I would imagine would be quite attractive to many applicants.
I would not look at nurseries. I would suggest you look at pre-schools (what oldies like me used to call playgroups) which run for around 3 hours as either a morning or afternoon session. From the term following your eldest child's 3rd Birthday, you would get up to 15 hours of funded pre-school education.
Richmond Pre-Schools Some providers are also Full Daycare Nurseries but have a pre-school element to them. This would give your eldest child some independence and your twins some time with your nanny, without older sibling around.
If you reduce your days to 4 - your nanny could still do 5 (if that is affordable). Surprising how much more you can get done without children in tow. Also means you can spend 1:1 time with individual children, taking them on a special trip out for example... to a cafe, to the shops, have their hair cut, children are not that fussy when it comes to having 1:1 time with mummy.