Clematis disease

My clematis 'helios' looks diseased - seems to have started at the base of the plant and is working its way up. In the early stages the leaves look spotty - almost like little pimples. The leaves which have been sufferering longer look black and decayed, and ultimately have died. 

If I google 'clematis disease' all I get is 'wilt' ... and I don't know if that's what this is. Any ideas?