In reply to Removed UserWhiskybob:
It works.
There are issues with the apple mice which mean that you can have big changes of magnification accidentally - very dismissive comment from the programmers when I made a request for help.
There has been no update to the program since 2013 (but is a 64 bit application). I think this may be a one time effort rather than a truly supported programme with a very small team.
The Harvey's maps (which I dislike having being brought up on OS) are fine. I got some to get used to some race maps that were using that format.
The alternatives are Ordnance Surveys own maps (no Harveys) or Anquet's offering which do offer the Superwalker maps. Anquet is less fully featured and does not have the good folder system down the side which is really neat, but is actively in production and the iPhone version (non-internet dependent if you preload) is excellent.
There may be others, but this is my experience. Hope this helps.