I held a straight edge (junior hacksaw blade) across the two facings and the shaft. The slightly lower faces of the old starter can be seen as the edge touches the (now proud) shaft.
Basically I need to lower the sides by about 1.5mm each side. With inspection, you can see the marks on the new starter where the excess part has fouled the flywheel on trying to fit it.
Further investigation reveals that all starters with nose cone casting number 54257610 have a slightly different casting, and fortunately that is the only difference.