Thanks for putting so much time into it just for me! I was just expecting someone to reply if they'd done it before or something.
I had the same idea of moving the controls points position to compensate. Some cubic bezier formulas have a weighting variable for each point - if the PolyBezier function had that it would be easy because you could just give the 2 control points half the weight (but it doesn't). I'm surprised there isn't a funtion for bezier curves of any order in the graphics dll. Maybe they can optimize it a lot more if it's just cubic.

Peter