One of the best sites in Essex is at Rainham as birds fly over the site to and from established roost sites in Kent (typical peak counts now well into three figures I believe). Most other reliable sites (based on records submitted for the Essex Bird Report) are in Metropolitan Essex where double and triple figure counts aren't unusual as the species expands eastwards to areas such as the Ingrebourne and Upminster as stated above.
Of interest, breeding first confirmed in Essex in 2004, with three sites in 2012 (but reported from another 20 or so during May-June).
Still v scarce here in the NE with two records from Frinton (if memory serves me right) since 2006...
Just realised the date of this thread - hence why the information seemed so out of date!!! Doh!