Thany you everyone for your feedback.

I've now planted the tree outside, at the top of a slope and backfilled the
hole with a mix of topsoil and sand, so it should be well drained.

We do get some hard frosts here over Winter, not sure what the lowest
temperature is though. I've had a brainwave (feel free to laugh). Over the
first Winter or two, until the tree is a bit more mature, plan to cover the
trunk with water pipe lagging - the spongy sleeve variety. The trunk is
straight, long and free of bits sticking out so lends itself well to this
insulation. I'll use largish diameter sleeving so air can circulate around
the trunk.
As for the top I'll wrap it in a little fleece. It will look like a giant
lolipop! Hopefully in a couple of years it should be mature and established
enough to do without any protection.

--
David
.... Email address on website http://www.avisoft.co.uk