Hello, we lived in west hampstead until a year ago, and had three shared care nannies during that time - very successfully.
I generally did two full days a week as its easier to get help for full days rather than half, especially if you want someone who speaks good English. Though our last nanny (who was actually more of a mother's help), did 25 hours flexibly over 4 days.
I've always advertised on gumtree, nanny job and childcare.co.uk, gumtree by far has been the most successful, but you do need to be very specific, but still you do need to wade through lots of unsuitables.
Have a look through some of ads posted by other people, basically you want to mention.
Non-smoker, professional childcare experience with children of a similar age with check able references, DBS/crb check (or willing to undergo one), valid visa for working in the uk.
You'll still be deleting lots and lots with no experience, but I've always had a good 20 suitable candidates out of maybe 80.
You will be paying £10/hour net, maybe less hourly if you want a full day.
The other place you could advertise is the library in west hampstead - the notice board next I the automatic doors is well read by nannies, though probably less so now that rhyme time has finished.
There are also sometimes ads up in the cafe in queens park (if you're that side f west hampstead).
Good luck with your search