Close to a tree isn't a great position for any plant, although Phormium cope with a fair bit of dryness. They like sun with a bit of shelter at their backs and where you live, don't plant it somewhere exposed to cold northerly or easterly winds.

Thanks Bamboo, so I guess at the end of my herbaceous border but near a few newly planted conifers that will eventually become a 5 ft hedge is not good either? I do have a spare tub would that be any good?

You can keep them in containers - means turning them out every couple of years or so in Spring and cutting them in half, then repotting, otherwise they outgrow their containers. Pot should be prefererably 18 inches deep by 12 inches wide as a minimum. If we keep having winters like the last one though, you'd need to move it somewhere very sheltered for the winter to prevent the compost and roots freezing.