I've done some more experiments to try and determine the source of the non-vertical walls I get cutting SRBP with single flute endmills. I surfaced the spoil board and trammed the spindle before running these tests.

In all cases the toolpath was a 25mm square in 10mm material, outer roughing profile to 9mm deep leaving 0.2mm for a single finish path at full depth.

#1 4mm Chinese SF, 0.2mm taper top to bottom,
#2 3.175 Chinese SF, 0.1mm taper top to bottom,
#3 6mm Euro SF, 0.03mm taper top to bottom,
#4 6mm Chinese 2Flute, 0.00 taper top to bottom.

The taper is not constant, rather it bulges towards the bottom where the skirt was left to hold the part for the finish pass.

I think that with this slightly flexible material the single flute cutters when slotting are leaving a slot smaller than the tool diameter, this is then cleaned up on the finish pass but not where the tool is cutting the skirt. The same material squeezing does not occur with 2 flute cutters. Has anyone else experience this with single flutes, I haven't noticed this happening in aluminium.