Robin,
You said: "It seems to me to depend on how your hand action is affected by the angle of the wrist".

RE:The individual range of movement by the wrist certainly varies a lot but that still ought not to be of great importance for concertina playing conditionally that the handle arrangement is fairly allright, and it often is not like I said. Firstly the wrist should be supported higher up than the trad handbar admits. Secondly the handbar should be sloping, higher at the little finger side than the thumb side of the hand. Thirdly for most individuals the handbar ought to be closer to the keyboard at the little finger side and for most hands more distant from the keyboard at the thumb side. This can may all be fixed by attaching a new hand rest by means of the common screws

You then said: " what you can buy off the self is a standard design of handstrap, so it is up to the individual to experiment if they wish to"

RE: The "standard design of handstrap" mostly is pretty useless. The handstrap ought to be steadier, broader and individually cut in a U or S shape that offers a steady support. Combined with a broader and better shaped hand/wrist support the handstrap may form a cuff that holds the wrist rather than the midpart of the hand to the instrument. By that arrangement the wrist can be flexed and extended and the fingers also in a more efficient and relaxed way. By rotating the handbar location according to the above you get space enough to add a thumbstrap to the Anglo/Duet handle arrangement and that helps a lot.

Naturally it IS "up to the individual to experiment" but it helps a lot analysing the situation at first to understand what you are actually doing and what you can expect from doing it. It took me more than 5 years going from scratch to a reasonable result and that was despite I am supposed to be an "ergonomic pro"....it is all selfevident once you've come out from the tunnel