Caper Spurge (Euphorbia lathyris). It's a biennial species which produces a regimented upright plant with four ranks of leaves the first year followed in it's second year with this branching top.

Incidentally, secretions from the roots of this plant are supposed to keep Moles away. Not sure of how effective that is.

We had it in our previous garden. The seeds dry off and explode and fling seeds about, so they pop up everywhere. I say off with its head. White sap said to be an irritant. You can hear them pop if your hearing is better than mine is now!

Yes, the three part seeds do "explode"/fly apart. A friend of mines garden in Northumberland had a large patch in a wilder part of their garden and you could hear them "pinging" every few minutes when ripe.

Thanks, that's really helpful!
No sign of moles anywhere nearby, so maybe this plant does help - although there's a rat burrow (I think?) just a few inches away from this plant in the greenhouse, so other species don't seem to be effective.