Thanks gebobs. I did manage to open the file today, (it took about 30 minutes), but it just froze when I tried to delete whatever may have been lurking in "blank" rows and columns. I had to close it down, but opened it again and was then able to copy all the data I needed into a new spreadsheet which is less than 1000th the size of the original, which I have deleted. I still have no idea what went wrong but thankful it's been sorted out.