There is no problem with overhang after all – I failed to spot that other overhangs were fine. The problem was that Slic3r was collapsing some walls down to 1 path thickness because the layer width wouldn’t fit at least twice.
Lowering the perimeter layer width solves it, but lowers print quality (had to go to perimeter layer width 0.6).
As a hack, I pulled the face of the stl to widen the wall. This made for a good print on the top corner, but more thought is needed if there is to be a solution for the bottom, because there is no room to widen as the conduit runs over it.
The red part is the successful top, the black is the failed bottom corner
Modified stl attached for reference.
EDIT: It wouldn’t let me upload the stl (security reasons)