I had found some of this plant, flowers spent and dull, dead brown, beside the filter beds, but it was also present, bright-leaved, beside the road to the car park - no flowers. After banging my head on several walls (i.e. making mistakes), and finding a curiosity of The Vegetative Key - having "parallel veined" entries on the opposite side of the key from "parallel veined (0-3)" (!) it did eventually run up a success, in part thanks to the excellent leaf illustration - Crosswort! Now I know I'll never miss it again because it's name's so perfect, but it took some time. Here's how it keyed out:
Lf margins entire
Lvs parallel 0-3 veins or palmate
Lvs whorled -> "M"
Stems square -> "MC"
Lvs 3-veined, whorls of 4
Lvs dark yellow/green, elliptic ovate, hairy -> Cruciata laevipes, Crosswort