After discussions with Raymarine have narrowed down problem to a single
Navionics Gold XL9 (256MB) card which cannot be read with v5.04 firmware. My earlier XL3 (64MB) cards display fine however.
Raymarine suggested removing a file chartcat.xml which the C series automatically adds to any inserted Navionics card to speed up display. When I tried this it just removed chart outlines with plain white background & display was as if no card installed at all.
Although Raymarine staff were understandably suspicious of this single
card, after re-installing previous v4.29 fw it displays fine.
I'm wondering if this is an issue with any Gold XL9 format card, but I only have the one.
Would like to hear from anyone with similar issue.