pjm, I’m not an engineer so I couldn’t tell you the angle of repose though I appeciate it varies in different soils.
At present, we both have consvervatories within 1 m of the each other, his 300mm from the boundary and ours 700mm.
We propose using the existing line of our foundations and have offered to underpin if necessary.
He by his own admission he has not consulted any professionals prior to throwing his toys from the pram.
What more assurance can I give than a legal document and the written opinions of three professionals?