Replying to myself here...
There were too many changes in the SVN plugin between 3.2 and 3.3.
I tried to revert some changes with no success.
So I took the first option: upgrade subversion to 1.7.
This was very easy. All works well and the Subversion functionality in Creator reappeared automagically after the upgrade.
I have to say the subversion upgrade to 1.7 was very easy after I searched the net for 'Ubuntu 12.04 subversion 1.7'.
Simply added a PPA, apt-get update, apt-get upgrade, svn upgrade in my checkouts and that's it. No trouble at all.
So the svn upgrade is the most easy and reliable solution to this problem.