Sue Says
It sounds like your Clematis is struggling a bit! The damage to the stems and leaves is likely to be caused by snails or slugs around May time. The easiest way to deal with them is to apply slug pellets around the base of the plant and hand pick them off the leaves and stems at dusk when they are at their most active.
The other common cause of damage to clematis is earwigs which chew ragged holes in the leaves and flowers, although this tends to be a problem later in the summer when the weather is warmer. Earwigs can be collected in small flower pots stuffed with straw. Place them upturned on a garden cane close to the plant. The earwigs like to hide in them during the day, when you can remove the straw and any resident earwigs and dispose of them.
We would also suggest that you prune out any dead stems now, to allow new grow to develop.