a bright green caterpillar with black horizontal stripes found on a Swiss Chard leaf
No interactions present.
That's a very distinctive looking caterpillar, but it's not one I recognise and a quick scan through Jim Porter's book hasn't turned up anything to match it. I wonder if the black marks could have been caused by a disease of some sort, they look a little bit like that, but I'm not at all sure.
Entomologist and biological recorder
Have just gone through same book and I would go for your suggestion of a form of disease,maybe fungus of some sort.
Lat/Lng: 50.7192, -3.363
OS grid ref: SY038919
in my garden