@Thiophilos and
@Woolsocks a friend has been telling me this since my first failure months ago. After this I did some internet searching and found some interesting information on why you shouldn't delete in camera. There's a lot of advice not to, but I found this the easiest to understand, from a former Director of Marketing at Lexar, Jeff Cable.
According to him, deleting an image doesn't actually delete it from the card, it deletes it from the FAT, (file allocation table) the actual image remains on the disk until it's overwritten by something else. Deleting an image from the FAT only removes the image file but not any ancillary files associated with the image file. Overtime the FAT gets scrambled leading to corruption. His recommendation was to offload your images to computer, delete them from the card, and reformat the card "in camera", before using again.
Something else I didn't fully comprehend is that formatting a card doesn't erase the data either, it just clears the FAT completely. Lexar supplies a recovery tool that will supposedly allow you to access that data even after formatting the card. I've downloaded it to try later today.