It's a few years since I was last at East Tilbury, but the lake in front of the fort had Ruppia maritima and Zanichellia palustris. A walk of a kiometre or so westwards along the coast path will see you at the old waste tips. These are very overgrown, but it is still easy to find Chenopodium vulvaria (several colonies on the paths and above the strand line) and Marrubium vulgare in abundance. The creek which dissects the rubbish tip carries locally plentiful Puccinellia rupestris and Chenopodium chenopodioides.
Regards, Tim H