Cannot access blogs at all - ERROR: 400 UNABLE TO SET AS REVISION AS CURRENT REVISION ON POST.

Hi,
I have just tried to upgrade from 5.09 to 5.13 and now my blog doesn't work properly at all.

I had an existing issue with the feed importer in that it os not importing correctly, I had some support with trying to fix this, and downloaded a file provided by Nik in an effort to fix it. It didn't work, then started importing loads of blank (1000+) posts all with the username in the title field.

I decided I had to upgrade to see if it corrected things. I got the above error, so thought clearing out these old blank posts, turning off feeds and deleting them via phpymyadmin (as I couldn't delete them from bog admin) I noticed they where all displaying the word legacy in the database.

I even tried creating a fresh menu link, now the new menu link at least opens the front page, but if I click on any of the blogs I get a white page after some delay loading.

I also had this issue when trying to upgrade to version 11 so reverted back to 9, but it looks like this problem will prevent me upgrading to ANY future version so i have to resolve it.

Please can you help me as this is very urgent!

Mel