Where is a good place for a family & friends get-together in London, UK?

I've got a few days in London in August (Summer) and want to invite about 20 friends and family who don't all know each other to a get-together somewhere neutral. I can't run around to see them all individually. Backgrounds very diverse from young lesbians to Aunties and Uncles in their 60s and per-teen kids! Could be a park picnic, a pub that's easy to access where we can 'settle in' for lunch thru dinner so guests can come and go at leisure. Help!