I've recently upgraded to Windows 8.1 from 8 and have noticed a Windows.old folder - this is pretty big (about 10Gb) and could use the space - would I have any issues in the future by simply deleting this folder?
That directory is where Windows 8 was placed. A Windows upgrade operation renames the old Windows directory to
Windows.old, so in case something goes wrong, you can rename that one and go back to the way things were. Obviously there might be problems with that, but you would only do that if you had problems with what you upgraded to.
If you are satisfied and ready for deleting the old Windows (not your documents, just the old directory for where Windows files existed), you should be able to safely delete it.
Read up on it at http://www.pcworld.com/article/228884/keep_win_old.html for more details.
You upgraded from windows XP right? Windows.old has all files that were used by windows XP. Although you can't boot back into windows Xp anymore, the files were kept just in case something could not be upgraded. If everything works as expected, you can delete it. Note that this folder does not just contain the windows files, but also your old profiles. This is for example where your mail was stored.