I Googled to try to find a resolution and so far have tried: 

1) Start > Run > outlook /resetfolders (and also outlook.exe /resetfolders) - did not work

2) Open Outlook > right click email account name > click "IMAP folders" > query > scroll to Junk folder > subscribe - unfortunately there wasn't a Junk folder option to scroll to.

3) Open Control Panel > Programs > Programs & Features > Find MS Outlook > Repair - MS Outlook did not appear in this list. There was only MS Office Home & Business 2010, so I tried a repair on this, but it did not work

4) Downloaded MFCMAPI and followed instructions as suggested by your Twitter support, but Junk folder did not appear in the list, so I could not 'hide' or 'unhide' it  http://www.slipstick.com/outlook/delete-outlooks-default-folders/

5) System restore - did not work

6) Deleted email account and added a new one - did not work

7) Swore at it a lot - did not work

So aside from uninstalling MS Office altogether and trying a reinstall, I'm stumped! 

I also have a second email account set up which has a Junk folder - but this is not the same junk folder as I had in my primary account. I think I can send junk email from my primary account to this junk folder, but this is NOT the folder the spam filter sends junk to for the primary account. As I've had a few non-spam emails automatically sent to the primary junk folder, I need to find out where it has gone.

Hope you can help.