When you say leaf mould, I'm assuming you mean mildew, a fungal infection. It won't kill the plants, it'll just make the foliage die back a bit if its a really heavy infestation. If its powdery mildew, then spraying with a milk solution will help - 1 part milk to 9 parts water in a spray. Mildew is quite frequent where plants have little airflow around them, can be worse in very wet weather, or, paradoxically, if the plants are dry at the root and the temperature is high. Usual answer is to thin out topgrowth to allow more air to circulate, but with honeysuckle, that's difficult. Some varieties are very susceptible to this infection too - you may have chosen ones which are.