I have found that installing a plugin sometimes does not work, this was possibly related to other users running it while I did this! In this case starting "MQExplorer.exe" with the "-c" (Clean) switch fixed this issue, there is also "-i" (Init).
Having helped a colleague out it seems iTunes stores phone backups on the C: drive of your Windows computer. If you have a small SSD or a small C: partition this can be a problem. A little bit of searching revealed the solution at Change backup location? | Apple Support Communities. In short you create an NTFS junction to the drive with all the space. Doing this moved 28Gb for my colleague, so was well worthwhile. If you make a mistake while doing this use "rmdir Backup" in a Command Prompt to remove the junction, of course having a second copy will give peace of mind if you fear doing that will delete your backup!