I got mine from John Taylor who does car covers. Not sure if he's still doing them but they were a straight swap.You need to give the arm a 'confident' pull out. Slightly convex, so ideal and with three screws you can get the mirror just right then tighten them up nicely. There are others on the market which I assume are also straight swap.
I bow to those who mastered the square types. I couldn't get them right even by moving the arm, slight improvement but no comparison with round ones which also look better IMHO. Not a huge outlay for peace of mind and relaxed overtaking etc.
