OK, here's a couple of images that show the way I eventually settled on getting the screwdriver blade to look pretty much like the original. The isometric wireframe shows the overall shape:
... and this second orthographic projection shows the critical part of the shape:
Basically, the blade is a simply disc, extruded out to form the shaft. This is then bevelled slightly with no inset to form the double-cut needed sharpen the transition to the blade.
Next, put a square in the background, centered on the shaft for reference, and make it slightly larger than the shaft itself, and take an even number of the points of the screwdriver shaft and stretch them in the X and Y axes, and drag them until they conform to the square.
Once this is done, drag the points (mainly) horizontally until they conform to the rough shape of your background reference image and perform a bandsaw around the upper part of this geometry to sharpen the curve. (this should give you the geometry shown in the second image above).
This done, simply extend all the points again, stretch them until they are all flat on the Z axis, and scale them down in the Y axis to form the taper on the blade. Finally, make the end of the blade by manually patching and bandsawing to fill up the area with quads (you may also want to add a knife along the edge to sharpen it up.
Finally, hit tab to subpatch.
BTW - I eventually ended up actually scanning the screwdriver at around 400 DPI to get a reference I could
really zoom in on to get the shape right.