The message
from "andrewpreece" contains these words:

Hello,

Further to my request for info on interesting specimen trees, I am now
down to four candidates; acacia dealbata, amelanchier, robinia pseudoacacia
"frisia", and the Rowan ( Sorbus Aucuparia ).

The Rowan, or Mountain Ash did not show up on the list of
suggestions offered in response to my original question, but I reckon it's
got
a lot going for it. I was particularly interested in the variety "Joseph
Rock",(snip)

Does anyone here have any experience of this variety? I should
appreciate an opinion from someone who has one in their garden, if they're
looking in,

Yes, I had it in my last garden near Loch Lomond, which was wet cold
and windy. It's a good choice, beautiful, tough and foolproof, and even
young trees set fruit.

It will stand colder weather and stronger winds than either the acacia
or the robinia.
Amelanchier is pretty and just as tough as sorbus Joseph Rock, but it
suckers, and all its displays are shorter (flowers, berries, autumn leaf
colour). I prefer amelanchier in a group of plants rather than as a
specimen.

Sorbus JR is easy to source from almost any garden centre, but it's
worth looking for one that's been well grown and cared for. Most of them
will be grafted. You'll see the graft "join" near the bottom of the
trunk, pick one that has a clean even looking graft union. If you get a
choice of sizes (small ones will be cheaper), I'd go for the small one.
They settle in much faster than tall pot-grown trees, and will catch up
growth within a year or two.

Janet.