How do I stop my macbook itunes continually needing reauthorising

I have a Macbook Pro running OS X Yosemite 10.10.2. I have recently (yesterday) upgraded iTunes to 12.1.

Now I can no longer access any iTunes purchases - I receive the error message that this computer is not authorised. When I try to reauthorise I am told I currently have 5 computers linked to the account. If I try to deauthorise all I am told that I cannot do that until 26th November 2015 (obviously I've had a problem before).

We only have two other computers in the house - my wife's (not authorised and never has been) and my son's (possibly authorised, can't check as it's at school). We have had laptops before - but having checked one of them (also not authorised) I don't believe this is the issue as none of the these have been accessed for years, and certainly not since the 26th November 2014.

I have tried everything I can find - the SC Info folder, modifying the Shared directory permissions using terminal, restarts, updates etc etc etc

I have checked the problem items - they are linked to the itunes store ID I can't authorise. I've also added the one other Apple account I have - no dice.

My itunes library has been forced on to an external drive - because of the size - in itself a pain in the ***, but it was working fine until iTunes was updated.

I am at the end of the line with other solutions and am stumped. One thing that would really help would be to know what 5 computers are linked to my account - but of course, that information isn't available to me - why, I have no idea, it's my account why shouldn't I be allowed to have this information?

Seriously, this is absolutely doing my head in. Any suggestions including buying CDs from here on, ditching iTunes and using a Windows machine are at the point of being acceptable.