According to Wikipedia the highest point of the County of Essex is Chrishall Common near the village of Langley, close to the Hertfordshire border, which reaches 482 feet (147 m). This is just west of High Wood. How reliable this is I'm not sure, but certainly not the Danbury Ridge or Langdon Hills, which don't come close to the heights in the northwest of the county.
Try http://www.ordnancesurvey.co.uk/oswebsite/getamap/ and GR TL424367 or TL443362