I found three this morning. They had already chomped half of a pot plant that the gf has been tending carefully, so she is well pleased. Also she has a genuine phobia about caterpillars, she went into the house while I rescued these. I had to prune the plant, their grip is so strong they wouldn't come willingly. Despite the gf's urging to squash them I relocated them in some forest nearby.
They really are tricky to spot, for something so big. The ones I found today are still small, I have seen massive ones.