Babysitting rates in Central London?

Help please, it's been 15+ years since I did any babysitting myself and now am in the position of needing a babysitter.  A friend is going to watch my little guy for an evening and I want to get her a gift card as a thank you, but I have no idea what the going rate is these days and I don't want to be the cheapskate who doesn't do enough or the weirdo who goes over the top.

Anyone?

Thanks!