Hi thanks Andy this helped
The boolean knife definitely does the better job however I get on the opposite side of the mesh a black polygon, first I thought this might be a reversed normal but that wasn't the case. After exporting to OBJ there seems no issue but inside AC3D it is visible.
Could I ask you to have another look ?
I tried to optimise but there were no vertices to fix.
Not sure what I done wrong again this time (I'm new to AC3D but getting used to quickly)
And another unrelated question if I may
how do I select edges ?
Polygons and vertices yes, but I don't know how to select edges.
cheers
AC3D file