Thanks all. I'm very aware that system redundancy is an important consideration - more baskets for the eggs. We have a small handheld marine GPS, compass, paper charts etc (no sextant!). I'm looking for a system that gives more or quicker or richer info with less risk of calculation errors.
I've been looking at Openplotter on a Pi device as an option - it looks interesting with lots of implimentation options you can grow into.
To Island Time and Steve's point, many aircraft (and sea-craft) have been lost because pilots thought they knew better than the computer guidance or control systems. Conversely, the 737 Maxx situation shows we need to be wary of systems so complex we don't know what they are doing. Software and hardware is usually reliable, its the dampware operating it that is in doubt!