Dear Graham
Yes, this certainly appears to be Phymatodes testaceus. I don't know its status in Essex, other than that it is recorded from both VCs including recent years, the books say it is fairly widespread in ancient woodland and pasture woodlands throughout Britain to southern Scotland, and the NBN Gateway and the 1999 BRC atlas show scattered records through the county (but mostly old). I would be grateful for full record details (date, grid reference etc).