Botanical name: Anagallis arvensis
Family name: Primulaceae
Looks like it could be a stringy piece of scarlet pimpernel, although I don't know for sure, it's not flowering yet.
Scarlet pimpernel is a relatively small, scrambling annual weed similar in size and growth habit to weeds such as scrambling speedwell and chickweed. It contains a few toxins which has occasionally caused poisoning problems. (Massey Uni Weeds Database)