I would tend to plant april/may Jon. Beware, they can be a bloody pest as they invade everywhere! They can also be toxic to animals, and keep children away. Personally I wish I hadn’t bothered putting mine in but you live and learn. I just took a cutting off a neighbour.
We have planted most of our garden between October and March when the soil is moist and temperatures are mild, that way they get their roots down before the stress of the baking summer months (still irrigate much of the garden through the summer). Lantana is pretty tough so shouldn’t present much of a problem.
Where we live at 100metres or so we get a frost approximately once every 10 years so it’s worth the risk, up in the mountains would be another matter.
They are not going to kill you stone dead on sight Martin. I use Lantana for very effective coverage, and Oleander because it is beautiful and makes a fast & efficient screen.
I have never suffered ill-effects from either, though I prune them both annually. Don’t burn Oleander, as the smoke is supposed to be harmful, though I have in the past and I am still here.
The many cats, badger, hedgehogs and pine marten that frequent the garden have never objected, as far as I know, and they all pass through both plants.
Many flowers that we use in UK gardens are poisonous too, you know. I would not advise using lily in your salad or lupin seeds in cooking.
I agree with Eleni. Unless you’ve got small children around, who might grab handfulls of leaves & eat them, they’re OK. As a precaution you can always wear gloves or wash your hands after pruning or handling them.