I would hazard an educated guess that you are over fertilizing the plant. Vitax Clematis feed is a slow release formulation, and lasts for 3 months. Feeding with Tomorite monthly as well is somewhat of an overkill, so I suggest you stop using the Tomorite and don't feed at all again this year, not even with the Vitax. Keep the plant well watered to try to dilute and clear away excess fertilizer. In the UK, plants in the ground really don't need much fertilizer and largely will grow perfectly well without any at all, though this is not the case in pots.
Clematis do best in neutral to slightly acidic soil in terms of soil ph; if you, or people in your area, are unable to grow blue hydrangeas, they're always pink, or cannot grow Pieris or Rhododendron in the ground, that would suggest the soil ph is alkaline - if it is severely so, clematis, whilst they might still grow, will not do very well. However, one of your other questions is in regard to Clematis Hagley Hybrid, and despite the presence of leaf miners, it otherwise appears to be healthy, so I'm not convinced the soil ph is too alkaline, assuming it is planted in the ground.