The plants are outside in a small raised bed. They're not in the sunniest corner, but I think i've been watering them enough. I think you could be right about the solution being too strong: I really doused the one that had the real problem and was a little more restrained with the the others. There's no black-fly now, but the one that had the problem is taking a turn for the worse - blotchy leaves and shrivelled flowers. That said, the others look like they're coming back. And the dodgy one appears to have given up on the main stem and has started a new offshoot from the bottom which is flowering nicely (if at a shonky angle). I'm going to pinch out the tips as suggested, and see where i go from there...

I don't know - it's all a little confusing. In other news, my first ever proper veg appeared today in the shape of some new peas that have somehow defied the ravenous snails to appear as if by magic. What a great feeling...