Hey guys, Right then, after a long and frustrating evening I have a 6 step fix! you just need a usb stick and some patience.
step 1. go into your hard drive and manually check to make sure all the save files have an owner; anything that says unknown profile, delete.
step 2. transfer all your data to the usb stick; some things wont move because you need to sign in to a profile, you can recover the tag to the usb and log in then move it. please note; its not worth worrying about DLC content...anything you have purchased/had a free code for, will be retrievable from the 'download history' menu in your gamertag profile when you recover it.
step 3. import your gamertag to the usb stick (it will work even though you cant download it to the hdd), look in your profile menu and use the 'transfer licences' function. you can only do this a couple times per year but hey, its not like xbox's die every day! ;-p
step 4. clear cache then format your hdd... please make sure you've transferred everything important to the usb stick and then taken the stick OUT!!
step 5. BEFORE you transfer anything back; recover the problem gamertag (any other tags will be imported from the usb when you do the save files and other content) as before, you may have to sign in to any relevant profiles in order to move some stuff.
step 6. ENJOY having everything back in its place... hello skyrim, I've missed you!
Good luck! I know its a pain in the proverbial but I did this by trial and error last night and have lost nothing except one fifa12 career save file because I wasnt paying attention when i transferred everything back to the hdd in step 5 (it wasn't from my profile and i didnt sign in and move it). hope it goes well :-)