Going rate for a 16 YO baby sitter for an evening (7pm - midnight)

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A neighbours daughter (16) has offered to baby site for us ( we are in Streatham) and I have absolutely no idea what the going rate is for a teenager to baby/house sit ( both kids will be in bed by7pm)

Can anyone help please?

Thanks

£5 an hour even if your DC's are asleep.

Thanks Lolwhut