Posted: 17/07/2015 at 07:58
It's fungal, Seb. Spraying with anything is pretty much a waste of time after the plant is infected. Spraying has to be preventive - before any infection. All you can do is remove all the affected foliage, destroy it, and keep an eye out for more.
Fungal spores travel on the breeze and there's no avoiding them if they're around. Early and Late Blight are both common to potatoes and tomatoes and can transfer. If you have spuds in the ground, check the foliage.
As Boater says, things like Blightwatch are only indicators of weather conditions that could be conducive to infection. If the spores aren't around, no problems regardless of the weather. If they are, they will settle on the leaves regardless of weather conditions. Dampness and humidity hasten their progress.