echinosum wrote:

I think M. umbellata can be much hardier than people familiar with the
much more commonly cultivated M. excelsa may realise, maybe it depends
on provenance of the seeds. M. umbellata grow at 1000m above sea-level
in the southern alps of New Zealand, for example in Arthurs Pass
national park, where I think they have real winter conditions.

They also grow in lowland regions, where the climate approximates that
of the warmest parts of the UK. As you say, it is down to provenance,
but better to be safe etc. etc. Winters can be very cold in the
southern parts of South Island, but even there the growing season is
somewhat longer than in many parts of the UK. Where plant hardiness is
concerned, tolerance of winter cold is only one aspect - duration of
exposure to sub-zero temperatures, length of winter, length of summer,
light intensities, rainfall patterns etc. are all factors that have to
be taken into account. There are numerous plants that withstand far
colder winters than we ever experience here, but are not hardy because
of those other factors coming into play.

It was about 60cm when I got it from Trevena Cross garden centre, and
is now about a metre, and looks like a big lollipop, or child's drawing
of a tree. No sign of it flowering yet though. I have heard that it can
take 20 years in the wild, though maybe in less trying conditions than I
give it, it would flower sooner.

Well tended, it can flower at a considerably younger age. When I lived
in the Midlands some years ago, I was given a seedling of this species
raised from some pods of seeds collected from around Dunedin. My plant
spent its first 3 years under glass, which was unheated except for
keeping frost-free in winter. It was then planted out in a sunny,
sheltered corner with plenty of peat and leaf mould and kept
continuously moist. In winter it was protected with a rokolene (green
plastic mesh) tent and heavily mulched to keep the roots from freezing.
Growth was quite rapid and flowers appeared a few years later. The
raiser of the seedlings had several flower young as well and one that
took 11 years to reveal it was the less common white form.

I have been advised to prune it "after flowering", which I presume
means about August if it doesn't actually flower.

No, I would prune in late spring if it doesn't flower ... except that I
wouldn't prune unless shoots were becoming very 'leggy'. This way it
has a whole summer and early autum for new shoots to mature (and
hopefully flower the following year).