Hi cordy,from my experience they really don't like root disturbance.I tried to rescue some self seeded plants in 2010,when I saw the really cold weather forecasts.They were a bit smaller than yours,but they didn't survive the transplant into pots.I know it's a gamble,if we get a harsh winter but I would try and protect them,as they are planted out.Best of luck with it.
if you do protect them make sure there is air circulation around the growing tip
mine rotted at the growing tip, branched out and I had loads of little flowers instead of one big one
I'm sowing each year, first winter in greenhouse, planting out in second Spring with a view to leaving them out for following winter and flowering in third season ... and expecting it to only work when we get a mild winter.
Perhaps a straw cage around them in chicken wire (is it feasible that such a thing could be lift-on/lift-off only in bad weather so the plant gets some light & air?)
I think I may have been lucky last winter (about 50 miles east of you Cordy) but I got two plants through in my front garden protected with fleece - 1 10m length of it. I drove 3 x 6ft roofing lat timber with end sawn to a point into the ground sloping around the plants and then wrapped the fleece over and round. I found drawing pins useful to keep everything in place in the full force of the wind and I think the timber helped to keep the weight of snow and ice away from the plants. 1 plant has been flowering since May and is around 12-13ft tall, the other had other ideas but is nonetheless a nice big architecural plant. If I can get it through another winter it might flower next year.
last year I had a 10-12ft flower spike from a 2 year old plant that was pot grown until planting out early June in it's second year. Another 2 plants have flowered this year which were in pots until early June this year, well into third year from seed and haven't acheieved anything like the height of the others so they really do need to be in the ground in year 2 to get good height.
The seedling from my old flower stalk have come up very late this year to last year picture 1 .I also have one large plant in picture 2 .Sometimes they go multi growing point and wont flower .
They survive some frost. There were some planted out at Wisley in Feb 2010 after they had numerous -8/-9C frosts. They were slightly under the canopy of a Eucalyptus but were still in good condition, and not mushed. I definitely think this is a case of only protect when necessary, or the cold dank damp will rot them.
I lost 8 outside in the cold winters we had before they went black minus 5 about it i think Kris.Like they say not easy to lift as they have tap root like a giant weed .
Here's a pic of the two I got through last winter planted out as they look tonight. I also found a seedling growing out of the patio earlier which is now potted up. Had a look for seed but not east to identify.