This national survey is ongoing.
The BTO's Heronries Census has gathered nest counts annually since 1928 at an increasing proportion of the UK's heronries. The aim is to count as many heronries as possible each year and to ensure that newly established sites are quickly discovered and brought into the programme of annual counts.
The main species covered is Grey Heron, but Little Egret is fully included, as are rarer species of colonial water birds such as Cattle Egrets (which nested in the UK for the first time in 2008). Nest counts of Cormorants are also collected, especially where they are nesting alongside herons. Data are shared with county recorders and for rare species with the Rare Breeding Birds Panel. You can download a summary of the 2019 UK season here: https://www.bto.org/sites/default/files/heronries-census-summary-2019.pdf
The following sites are currently being monitored, some of which are actually in Essex (Lea Valley) but come under the Herts BTO region.
Nether Hall GP
Holyfield Lake, Lea Valley CP
Waltham Abbey Woods, Perme North
Stanborough Lakes nr WGC
Wilstone Res (Tring Reservoirs)
Frogmore Lakes nr Radlett
Tyttenhanger GPs - main GP
We are already into the 2020 season for these early nesting species. If anyone is aware of Grey Herons, Little Egrets or Cormorants breeding at a site not listed above, and would like to get involved with the Heronries Census, please let me know firstname.lastname@example.org