back in the UK now, and bought some warm weather with me from the middle east.
Drying out the card does not seem to have made any difference. When I look at the menu item on map data, it only has "Lawrance' as an option. No option for NauticPath. And I can only see one SD card slot.
When I look at the SD card on a PC, I can see three files there. The largest is called NP-EURW.at5, and is around 95 Mb. Then there are two other smaller files, called International Transas Currents.at5 and International Transas Tides.at5. All the files date from 2008.
The Navionics web app on their website seems to give detailed charts without having any card plugged in. I have Navionics on my iphone and Nexus 7, and it works really well.
A real mystery. I don't want to buy a new SD card, and then find out it is the plotter unit. Don't suppose there is anyone local to Thame / Aylesbury that has a known good SD card I could try in the plotter.....