Your Buddleja may be suffering from fungal leaf spot which is caused by thousands of different fungi species and affects a wide range of plants. Fungal problems tend to be exacerbated by wet, humid conditions. The recent warm, wet weather that we have experienced have provided ideal conditions for the growth and spread of fungal spores which are dispersed by wind and rain splashes .
There are several measures that can be taken to reduce this problem. Firstly try to water the plant directly into the soil, from below the foliage, as this will help to keep the foliage drier and reduce the spread of spores. Cut back the infected areas and destroy the infected plant material by burning it or by disposing of it off site. Do not compost it. If the plant is in a container then move it to an open, well ventilated position. This will help to dry the foliage quicker following rainfall, and reduce the dispersal of spores. Any fallen plant debris should be raked up in autumn and destroyed to prevent spores from overwintering in the decaying foliage. This will help to prevent further infection in the spring.
There are also many fungicides available that are suitable for use against leaf spots on ornamental plants, and these can be used to help prevent further outbreaks.
A healthy plant is less likely to succumb to diseases so the avoidance of plant stresses such as drought will also help to control the problem.