I recognise this, and it does pop up as either a wild plant or a weed, depending on your point of view, but I can't for the life of me remember what its called, so I'm adding this comment so I get to see the answer when someone else names it!

Can never remember the proper name myself. but it is a Euphorbia and a biennial so once it has flowered it will set seed everywhere and die. Personally I pull it out as soon as I see them, carefully because the sap is very irritant.

Could it be Euphorbia lathyris (Caper spurge), considered by many as a weed. If you copy and paste into google images there are plenty pictures available.

Gosh, thank you all very much! I think I too will class it as a weed, but what a shame as it is so striking and has taken to years to get to this point!
Anyway, thank you all!