Thanks for the suggestions bluebear and vix1.
I've used agencies before, but not Simply Childcare, so will get in touch to see what they have to offer.
Anyone had any recent experiences of After School Nannies?
I know some people also use Gum Tree, but I am leery of dozens of applications from unsuitable types - and quite honestly have felt happy to have an agency put forward suitable candidates once they have checked references, done police checks etc.
I am fine with nanny with her own child, but in the past, I have had difficulty with the perception from nannies that it's too far to travel (live in London zone 1) - or, at least they think it is. Many told the agency they didn't even want to come for an interview when they realised it was a train journey into town - and then it's also difficult for when I want an evening babysitter (usually once a week, but still not easy IME) as nanny mums understandably want to get their children home to bed.
As far as a nannyshare, I don't know anyone who does it (most of my acquaintances seem to hire housekeepers who do childcare as/when needed and that's not what I'm looking for), but it must exist in these parts....just that I haven't come across it.
Still, thanks for these suggestions and I will pursue various options. And to think that 24 hours ago I was looking forward to a long, relaxing summer break followed by an easy transition into the school year.....