I think Mother Nature will probably rule over the outcome.
Well, it didn't take long, I forgot about the magpies. They have obviously been watching and came twice yesterday to bully the mother pigeon. so sadly, I have removed the nest. Too close to house to listen to the distress of eggs or babies being taken.
Pigeons nest 6-8 times a year, so they will soon be building again.
Honestly, I thought this thing was a slug but my local museum have identified it as a species of Flatworm called Caenoplanabicolor. I hope they can identify the caterpillar, too.