A quarter of a tonne Agave, that must be something Rupert! They can
take a fair amount of cold, but need to be quite dry at the root. For
any of the hardy Agaves to survive, extremely sharp drainage is
essential, which means digging in a barrow-load-full or two of grit
into the soil of a raised bed.

In colder parts of the country, plants suffer badly from 'radiational'
freezing on clear nights in winter, so fashioning some sort of
open-sided tent over the plant is necessary. This will also help fend
off any rain, which can damage plants if it is followed by near or
sub-zero temperatures. To cap it all, it needs maximum sun during the
day. Away from the south coasts, Agaves are best planted where they get
high overhead cover from tall evergreen trees, but can also see the sun
for most of the day. Not always easy to organise, so artificial covers
are the way to go.

As to those needles? Yes, you can take their tips off, but do not cut
into the fleshy part of the leaves otherwise they will die back. On
this last point, I've recently brought back a 25+ year old, 5ft. potted
Yucca aloifolia (Spanish dagger) from my mother's house in the
Midlands. For some years it has stood outside in a sheltered spot and
mercilessly stabbed anyone who came near. I tipped the leaf spines
last summer, but it was decided that it was still too dangerous to
remain since the leaves themselves are very, very hard and the fine,
saw-teeth along their edges can inflict deep lacerations.