so I have a backup - essentially a repeat - of the map on my small screen in easy sight of the helm, with the big map for the navvigator - it is hard to argue that is a breach and it certainly seems unprosecutable....
That would not be a backup of the software.
If both were in use at the same time it would be very easy to argue that it is a breach of copyright & therefore would be prosecutable, but unless the copyright owner wanted to make an example of you, it is unlikely they would prosecute.
It would depend upon what the license says, but in general there are 2 types of software license:
1, The media can be copied only as a backup but not used.
2, The software can be installed on 2 (or more computers) provided that the computers belong to the same person & are not used at the same time ie a desktop & notebook. This is often know as a Borland license.
The 'shrinkwrap' license that covers most software has never been tested in an English court, the outcome of any test case would be extremely difficult to predict & the big software companies don't want to take the risk of a decision going against them.