At the very bottom £7 gross for someone totally unqualified and inexperienced right on the outskirts to double that and more in zone 1/nanny hotspots for someone qualified with 10 years experience working normal hours or some less qualified/experienced working extended hours.
I believe that 'average' costs are £450 per week excluding taxes that you have to pay on top (net to nanny), £595 a week including taxes (gross to nanny), £659 including employer taxes (gross cost to YOU).
Plus costs for travel/activities of approx £20 per week (at least)
That does cover experienced and unexperienced people so there is probably a range of at least +/- £100 there (£350 for inexperienced, £550 for 10 years+ experience)
This link is great (thanks MrAnchovy) for calculating gross from net.