Mystery plant from London brownfield site

Mystery plant from London brownfield site

Observation - Mystery plant from London brownfield site. Description: Really stumped by this. Flowers in back-to-back pairs with 5 tepals and a single bract (of similar appearance to the tepals) beneath the pair. Just intrigued to know what it is and frustrated not to be able to work it out. Have tried to key it in Stace I

Really stumped by this. Flowers in back-to-back pairs with 5 tepals and a single bract (of similar appearance to the tepals) beneath the pair.
Just intrigued to know what it is and frustrated not to be able to work it out. Have tried to key it in Stace III's Amaranthaceae key but didn't get a sensible answer, which makes me think I've guessed the family wrong.