Hi Phil,
In pretty much the same boat as you, I live in Loughton.
I have seen people flying over Hainault park but I'm not sure that the powers that be are keen on kites.
The Roding Valley flats (along the river Roding towards Debden) are quite good. Often a few up in the air down there and theres a fair amount of space for a decent run of the board.
Give me a shout if you find anywhere else.