Thank you all for your input. I appreciate it's hard to diagnose things like this using just photos.
The fertiliser used was 16-46-9 if it helps.
What other information can I give to help you?
It hasn't been sprayed with anything recently.
Watering is applied directly to the soil, so no splashing on leaves.
This plant is growing outside in southern africa, so it does get rained on, but the raindrops are big.