Hi Rhys
I have had the same problem although my plants have started forming pods. I SPRAYED using very diluted Ecover washing up liquid but repeated the spraying each day for 3 days, targeting the actual blackfly rather than a general spray. You do need to get in close. This did the trick. However I will continue to do this every few days as it is easy to miss some and the ones missed seem to make up for all of their relatives who haven’t survived!
John