Welcome fellow Essex resident
Im down in the South west Essex so may be a bit far away. As recommended beforeHanningfield is good for birds (and grass snakes I hear, never been there at the right time of year) and nearer to you, but down here we have RSPB Rainham -which is great for birds if you have a long lens/digiscope set up, but if you like me with less range planty of marsh frogs, water voles and inverts to keep you happy, and I have seen weasels there too. My local park, Bedfords Park, Ive seen foxes regularily and some stoats and a black rabbit, plus butterflies and dragonflies. Go to the Essex wildlife trust site, as they ahve loads of good reserves scattered all over the place