Sorry Mart, can’t help you much there.

There’s this from English Heritage which, given the ‘explosive growth’ in salt production prior to the Roman invasion, suggests there might have been (probably were?) barrow complexes in the area as well.

“In all likelihood salt-gathering and salt-making predate any archaeological information currently available in Britain. The earliest actual evidence for deliberate manufacture has been found at South Woodham Ferrers, Essex, dating from the middle-late Bronze Age (1412-1130 BC); and a handful of other, slightly later sites have been discovered in Lincolnshire, Somerset and Kent. The early Iron Age is better represented, with a substantial number of sites known in Lincolnshire, East Anglia and along the south coast, but this was followed by explosive growth in the last century or so before the Roman invasion of AD43, related to the introduction of the open-pan process.”

Only other thing I can suggest is you try contacting Nick Wickenden, Museums Manager at Chelmsford Museum. Details here - http://www.chelmsford.gov.uk/index.cfm?articleid=6804