having a look around the web I think this is just a case of aphids / green fly.

any tips on the best way of getting rid would be appreciated.

Aphids can be controlled with daily strong sprays of water and smushing. Neem oil is effective, too, as is insecticidal soap. I think I'd get rid of as many as possible with the spray and smush method, and then apply the neem.

Neem acts as an anti-feedant.

On only two plants you should be able to just wash them off and squash as many as you can with your fingers. Also you could try to find a few ladybirds and transfer them to your parsley.

thanks.

I've washed most of them off now. I'd love to get a mob of ladybirds onto them but I haven't seen a ladybird in ages!

Some websites suggest spraying them with a very weak washing up liquid mix. Others suggests pyrethrum.

is this just over kill? literally.

I am an avid lover of caterpillars and I had two very small (less than 1/4 inch) long Black Swallowtail caterpillars on my parsley. I was hunting for more and found little tan and brown speckled critters with wings sitting on my parsley seed heads. There must have been at least a hundred. I have no idea how to get rid of them without hurting my precious swallowtails! Any suggestions would be greatly appreciated.
Thanks in advance!