I thought I should mention this, as it may save some fellow RIBnet participants a few moments of concern ...
I updated the software in my back-up system yesterday, using the December download from the Lowrance US website.
I was a little alarmed when I replaced the Navionics card and found that my local area was nowhere to be seen!
On checking the manual at home today, it looks like I need to tell the GlobalMap which charts I am using, otherwise it is almost certainly looking for a Lowrance chart. I suspect that the update restored default settings for everything.
Hopefully, this is easily resolved - what a pity that my boat is 100 miles from home!