iTunes backup location - how can I change it?

I originally had iTunes installed on my (tiny) C drive, so re-located the media library to my much bigger D drive.
However, whenever I want to sync my iPhone, the backup still seems to be done to the C drive.
Given the size of backup it's trying to do and the lack of space on the C drive, I would much rather it did this to the D drive, since I have to stop any backups currently due to not having enough space on the C...!

I have tried re-installing iTunes to the D drive, but this hasn't stopped it from using the C for the backup.

Can anyone help advise what I need to do to fix this? Am I being completely thick, or is this just part and parcel of 'wonderful' iTunes?!?