Thanks MB, I'll have a look at Bradfield & Harwich. I found several at Dovercourt when I was looking for something else. Have you read "Martello Towers & the defence of N.E.Essex" by Kenneth Walker 1938, there's a copy at Essex Uni, only 14 pages but there may be a few new things for you. It was originally in the Essex Review. Amazon had a copy but it was sold.
Best, A