Tristram - My experience is the same as Gill's - and I'm gardening in the north-west of England, not in France (though admittedly right by the coast). Here Acnistus australis, which has now I think been absorbed back into the Iochromas, has survived outside against walls for many years (many = a decade or more). It seems to do rather better (more lush foliage, more profuse flowers and seed) in warm moist semi-shade than in full sun. And I agree with Gill, too, that it's perfectly easy from seed (the problem is separating the seed from the sticky mass in which they're embedded) - and that seed will usually throw up both blue and white forms and occasionally the pink form as well (though all the ones which have appeared here, or which I've seen elsewhere, have been a rather muddy pink and not, to my mind, worth growing).
One thing I would add is that in its first year, when it's still soft and sappy, it probably isn't
hardy. Or wouldn't be if you're gardening in a place which gets sustained spells of frost, at least. So I'd suggest growing seedlings on until they've made some woody growth and planting out at that stage. The final few inches of soft new-season growth may then get killed back over winter but the plant reliably shoots again from the hard wood.
In Cornwall they grow several other and even showier species of Acnistus (or Dunalia or Iochroma: take your pick). I haven't tried them up here yet but an equally optimistic friend and I have an order placed for some young plants from a nursery on The Lizard. It'll be interesting to see if they, too, prove to be distinctly hardier than most of the textbooks suggest.