If the museum is fortunate enough to acquire it, I will seriously consider including it in the new galleries planned for Colchester castle, and scheduled to open at Easter 2014.
Terrets of that kind are very elaborate and ostentatious with the red enamel (glass) inlay. It is certainly late Iron Age, say c. 100 BC- AD 43 +.
Iron Age terrets are a real puzzle in Essex. Unlike Suffolk and Norfolk, they are pretty rare; and no one really knows why. My preferred explanation is that many of them down here were iron or wood. The iron rusts into oblivion of course, and it is therefore difficult to recognise them when they turn up in the plough soil.
It is likely that pieces like yours reached the county from elsewhere. I like to think that rather exuberant pieces such as Mersea might even be relics of the Boudican war host that ran amuck here in AD 60. A long shot, but it is certainly not impossible.
All the best
Paul R. Sealey