Mike Lyle writes

Well, it doesn't do to be complacent, and it seems I was. But I note
from the site mentioned that its typical native habitat is dense
sclerophyllous woodland: I don't think there's too much of that in
Scotland. It's presumably significant that the plant has become a pest
in dryer zones, but doesn't seem to have colonised non-Mediterranean
Europe, though it's had since the last ice age to do so. It's also had
ample time to escape from British gardens and arboreta, and has indeed
established itself in some places, but I haven't heard that it's
present in overwhelming numbers.

Jap knotweed and Rhodo ponticum (and, in America, tamarisks) have found
conditions similar to their home range but geographically isolated from
the competitors, insects, and diseases which keep them in check there.
P. pinaster hasn't had these advantages in the British Isles, so on the
whole I don't think Roger should go around digging out his specimens.
Even if the climate continues to warm up, there are far too many of
these trees already established for Roger's to make any difference --
and anyhow, in those conditions they'd be likely to colonise naturally.
But I repeat, one shouldn't be complacent.

Absolutely. It's easy to feel that the UK flora isn't that interesting
and that introducing foreign imports can only do good and brighten
things up a little. But in some areas, what we have is of international
importance - bluebell woods, ferns, mosses, lowland sedges, for example.
--
Kay