It's at the Barn Church in Kew on 22nd September. There's an event listing on the right of the page with more info.
It's not table top so you can sell just a few bits or loads without paying a hefty upfront fee you might not see again (and you're more likely to sell your stuff because shoppers can find it). Registration is £5 and includes your labels and all admin costs and then you pay a commission on anything that sells. If you volunteer in some capacity it's 25%. If you just want to leave your labelled items and go it's 35%.
Online registration has now closed (it's a fancy pants automated thingy) but if you email [email protected] I don't think the last sellers packs have been processed so we could probably squeeze one more in :)