ST1: Removed several rocks in the northern half of the trench, now it appears that those rocks were part of, or sat on bedrock. Taking the north half of the trench down hoping not to find bedrock there too.
No bedrock fortunately but a wall. Having defined the wall, we've extended the trench 4m further south, and are now clearing the sod topsoil. The new extension just catches the interior corner of the wall that is visible on the surface that ATS thinks is late.