Recuva worked fine and detected the excel file I was looking for but sadly said the file was 'unrecoverable'. Of course I asked it to recover it anyway and it gave me an excel file which could not be opened. However the file is 18 kb so is it likely to contain some of the original data or will it contain a fragment of what overwrote the file when I reformatted? Is is worth trying to open the file?
Unrecoverable means that all (or virtually all, I don't know the actual figure) of the file's clusters have been overwritten. Recuva will recover whatever is in those clusters. The recovered file will contain whatever overwrote it, and will be of no use in reconstructing the original deleted file. In advanced mode you can see what clusters have been overwritten, and by what, in the Info pane.