Found this poor chap this evening as I was doing a bit of watering. I believe it's a Black Whip Snake (Dolichophis jugularis) that can grow to 3m in length. They don't take on the black colour until they are around 5/6 years old. They are not poisonous and are therefore welcome visitors as they take rodents, some insects, and even other snakes. Many years ago, before the land around us was built upon as much as it is today, we used to get very large blacks but I haven't seen a big one for a long time now. The secateurs are for comparison.