Perhaps member "Thecat" has the answer to that since this member has grown clematis in pots.

Where possible, a thick pale-coloured earthenware pot will be cooler on the roots than a darker or thinner material.
A topping of slate chips, mini cobbles or similar will shade the soil, and collect condensation overnight to keep roots cool.
Even shards of broken pot or roof tiles laid over the compost will offer some benefit.

thanks for the input, I will have to monitor the sunlight in mother in laws back yard discretely, as I plan to make her a clematis planter/support combined as a surprise pressie [its quite a sun trap, but totally paved]
cheers again kev