Well that depends on the nanny/mother's help.
There's often not much difference between the two in terms of pay unless you have a super-experienced nanny or a very inexperienced mother's help.
Reading your other post you want a s/c nanny and then shared-care with more household stuff thrown in so I think you're right to look for 2 different people.
A nanny could be anywhere from £9-13gross, a mother's help £7-11gross.
The nanny salary will depend on their qualifications and previous experience - someone with a qualification and 2 years experience might be on £10gross, someone with a qualification and 10years experience is more like £13gross. Unqualified nannies on average seem to earn about 50p-£1 less than qualified nannies with similar experience until they have so much experience it really doesn't matter any more and in S. London that usually means £13+gross/hour.
Mother's helps often have a wider payscale because at one end you get inexperienced/newly qualified younger candidates who are looking for experience and are willing to do more to get it with a bit less pay. IMO this sort of candidate is ideal where there's not too much housework (as they might need to be shown how to do things) and plenty of shared charge building up to sole charge gradually. Then they can often be kept on as a sole charge nanny with a reduction in household duties at the end of a maternity leave. They get around £7-8gross.
Another kind of mother's help is an older lady who knows what needs doing and gets on with it - equally capable of doing childcare and household duties. These can be more expensive - £9-10gross.
The third type of candidates for mother's help jobs are ex-au pairs who are too experienced for au pair jobs but not experienced enough for nanny jobs and they're in the middle pay wise at around £8-9gross.
I think your tentative hours could well suit a local mum with a child at school or someone with an afterschool nanny job looking for newborn experience. Have a look at candidates on Gumtree, your local netmums and nannyjob and see what they're asking for too.
I'd always advise working backwards from what you can afford, bearing in mind that you will need to pay employer's NI and provide a kitty for activities/misc. expenses, adjust your heating, light and food costs and various other hidden costs e.g. if you go through an agency that can be 5x nanny's weekly salary. That way you don't meet someone you love and realise you can't afford them. It also means you've got a realistic payscale to put on an ad (but leave a little room at the top to negotiate with exceptional candidates).