New greenhouse - this may take some time and you might get bored

If you'd put a thick layer of those stones down at the bottom of the trench and covered them, the stones would warm up and act as a heat store.
Could you share his reaction when you suggest that he should dig it out again.

Having done so.........this could be my last post as my tablet might go flying.  If the trenches were a bit deeper I might have considered it but at less than  20 inches before solid stones, subsoil and chalk, I think I'll let the layer of stones idea go.