Hannah at SkyDemon has sorted this out for me. The procedure was to delete the charts from the SD device, do the same for the PC and then download a new chart to the PC and sync.
Appropriate part of her message follows:
To delete charts from your Mobile SD you'll need toconnect the device to your PC, then click Computer and select your SkyDemondevice and open the Resident Flash drive. You will then need to open the SkyDemon folder and inside this the Datafolder. In SkyDemon Data you will see anumber of .skydata files, each corresponding to a loaded chart. You will need to delete each of these skydatafiles.
Next you should delete all of the .skydata filesin your SkyDemon folder in Documents on your PC. This means that the next time you launch theSkyDemon software on your PC you'll be prompted to install new charts, and Iwould suggest installing only the ones you need.