Plant ID! What could this be?

Ian142 says:

OK, anyone know what this young plant/weed is? Looks a bit like a euphorbia (we have a couple in the garden but they're different to this i think) but i think it's maybe a weed. Got a few coming up which have seeded around the garden, been pulling them but i was curious what they are so i've left a couple to grow.

ignore the dianthus leaves in the background, just interested in the vivid green taller thing at the front.

many thanks!

See original post