I took the hint - after almost a week of trying to get CB installed into my RC3, I just gave up.

Instead I wiped the Joomla RC3 installation off the face of the earth and started with a clean install of 1.0.13.

CB 1.1 went in clean as a whistle BUT only when I used both the readme.txt and the installation pdf (which BTW are not 100% interchangable) to guide me.

From my experiences this week I would strongly advise against installing CB into a 1.5 RC3 set-up. Better to wait until Joomla 1.5 goes stable.