Kay wrote:
It seems to be biennial in habit - if doesn't flower the first year, and
it dies in the second year after flowering

Ah yes, I forgot about the north-south divide. In the south, seedlings
germinate quickly and make good progress often continuing to grow
throughout winter. These usually flower in late spring and die as the
seeds ripen - hence the annual bit. Further north, the seedlings make
much less headway, barely reaching more than a couple of centimetres
before winter and many do not even germinate until the following
spring. Most of the growth takes place during summer and with flowers
developing in the spring of the next year.

Quite right about the 'caper' bit. The common name merely alludes to
the similarity that unripe seeds have to capers. True capers are the
pickled flower buds of a South African shrub - Capparis spinosa - which
are pleasantly piquant or not depending upon your taste. Try pickling
and eating the seeds of Euphorbia lathyrus and you'll end up feeling
extremely unwell.