"Pete C" wrote in
:

UPDATE
On further inspection today, I found it on swede and strawberries too.
It all washed off with a watering can. I checked half a dozen plots
either side....nothing. Then I realised the affected beds are all
adjacent to paths. Getting paranoid now....who have I upset?

I doubt that you have upset anyone. It IS puzzling though.
Let's see what happens in the next day or two. Eh?
Could it be as Sacha said, Bordeaux mixture near your plants?

Baz