The location for Sundays picnic is at the Lily pond in front of Kenwood House, Hampstead Heath. http://tiny.pl/f8bz
We should be there from around 12.30 so turn up at any time after that.
Bus routes that go to or close by are 24, 46, 168, 214, C2 and C11. Trains, Silverlink to Hampstead or Gospel Oak. Tube, Northern line to Hampstead high street.