Thanks for the responses. Having investigated further I'm reluctant to go the pressure treated spruce root. Apparently the new treatments used, even though better on the environment, don't penetrate the timber so well. Regarding larch, without using creosote (which I won't do) the hybrid larch will have a significantly shorter life in the ground. In the end I've chosen to for with untreated chestnut. It's double the price of the spruce though should last a long time in the ground untreated. It's not a job I wish to do again if I can help it.