Hi Nina,
firstly it isn't a Clematis montana it looks more like the C armandii type, probably an evergreen one. [was it bought as a clematis? I ask as I have an evergreen honeysuckle that does look like this from a distance]

as for what is eating it that is hard to tell. there are several moth caterpillars that will eat it. But the roots are also in full sun and in the wild they have shade at the roots so that might be a problem. or not!

bare stems at the base is also common as the plant grows along the fence. how tightly are the stems tied onto the fence? perhaps they could be a bit slacker to allow the nutrients to move into the developing shoots.

Hi, thanks for helping.

Yes - You are so right, it is a Clematis Armandii!

The stem isn’t tied to the fence at all, the base gets quite a lot of shade, I wonder if I should pop something over the area where it’s growing so the soil keeps cool?

sorry it looks as if it is tied up to the fence is it trained around a wire?

putting something at the base to shade it wouldn't hurt and may infact help it to retain moisture.

Someone on here recently, suggested water retaining crystals could help with keeping plants hydrated, could it help with this clematis? It could have even been you Seaburngirl! I'm about to try it with my Clematis Prince George which is ailing this year.