It sounds like downy mildew. Mildew likes mild summers that are wet. Like the recent weather. When you run your hand through it you get a horrible white powder on your hand. You need to thin out the plantings around the infected Busy Lizzies and burn/bin any infected plants or it may spread to other plants in your garden.

many thanks for your advice

The best summer ever for Biz Liz was a very hot one we had some years ago. I had planted my elderly neighbour's front window boxes with BL and they bloomed their socks off. The sun was off them by 11am. Provided they were watered and fed they were pleased to be alive and well. They even seeded and flirted their seeds around. This wet, wet, wet summer does not suit them, so it seems.

thanks for your advice