Typical. After posting here, I've found the problem. It was restoring the files to the wrong place.

All the data was being put in a directory called 'archives' rather than 'accdata'. Copied the files across and it works fine.

I'm guessing it was something to do with the age of the data - the backup file that had worked before wasn't working now either, so maybe Sage decided it would archive it or something.

I'm so glad I don't have to use this software every day.