Thank you for the suggestions. The parsley is in several pots on my patio in a sheltered shaded position (it's the only area I have outside the flat). It's green, healthy, and growing well.. but some bits are yellowing in the lower area of the plant. No ants around either, so I won't have to deal with that.
Sounds like I'll just have to go on a major attack and squish as many as I can before doing lots of spraying. I'm noticing some on my lettuce now.. so I need prevent them going any further.