1stC BC Corieltauvi celtic stater CCI 07.0103
'many thanks for the images, this is very interesting. It's your first Corieltauvi - to be precise the kite stater, listed as VA 825, nos 3181-3184 in the BM catalogue and no. 392 in 'Coins of England'. The reverse needs to be rotated 180 degrees and you get the horse left, with the diamond-shaped kite above containing pellets; and faint remains of a wreath design on the obverse.
This type has become relatively common in the last few years because of some substantial hoard finds in East Yorkshire - something like 150 recorded now. It's fairly unusual to see one down in Essex, although there are a few Corieltauvian coins from the county. In terms of date it's probably very late first century BC; most of these coins are either in base gold or plated. All in all a very interesting addition to your finds. I'll record this as CCI 07.0103'. Dr Philip de Jersey