Hi eeyore
I live in south Essex and I can recommend these.
Hanningfield Reservoir
Abberton Reservoir
Fingringhoe Wick
Wat Tyler Country Park and Vange Marsh .... my local patch .... see my
blog.
All of the estuaries and marshes, particularly in winter for their ducks, geese, waders and raptors.
And probably all of these! ....
Essex Wildlife Trust reserves
Richard