The Scarce Emeralds at Wat Tyler are on the small pool to the right of the entrance track, approx. 100m after the entrance gate, i.e. before you get to the cottages on the left.
Probably the easiest place nearby is at RSPB Bowers Marsh, just east of Wat Tyler. Take the track from the car park, turn right after c50m, past the barn and the picnic tables. Just before you get to the first metal gate there's a small pond on your left, maybe 15m from the path across a recently ploughed strip, they're in there.
A few Southern Migrant Hawkers have also been seen here, as well as at the nearby RSPB West Canvey Marshes in apparently good numbers.