The contractors have now agreed with me and have kindly agreed to put it right. I was expecting a couple of men carrying spades, but they had other ideas.
They turned up early this morning with two enormous machines and the first thing that they did was remove the big pile of clay that was in their way.
I was so pleased because I was planning on hiring a skip and loading the clay into it by hand, but within ten minutes it was all loaded up and gone.
They then dug a huge trench and our ducting is definitely now deep enough, so there is no chance that it will freeze in the winter.
What a relief, and a big thank you to the lovely workers.
Wagtail update - it has been a very noisy and busy week in the front garden, but they have ignored it all and spent all day every day bringing tasty treats to their babies.