Euphorbia lathyris Caper Spurge C DD N?
This is another very large spurge - you really can't miss it. Having the typical spurge type flowers means that you've narrowed it down to one of only a few of this genus. Doubtfully native in the south of the country it was certainly introduced at this site where many other oddities grew. The seeds contain an unusual proportion of oil (50% by weight) so that the plant has been suggested as a possible bio-fuel crop. Trials have already begun with some success in Germany, the US and here in the UK but I doubt that was the purpose of the introduction at this site.
E. lathyris is very common in south and central England and in wales away from the hilly centre. As you go north records diminish and it becomes uncommon in Scotland. It is present in Ireland in patches in the north and south without much in the centre or west of the island.