I have finally succeeded in beating this challenge following the outline provided by Ryan Munnis elsewhere in this thread.
Every time I tried to install Rapture I got a message telling me that I needed to upgrade Dimension Pro to version 1.5 (which had already been done) and Rapture never installed.
I deduced from this that there was at least one entry in the registry which I had not temporarily re-named to allow the rature installation to proceed. So it proved.....
Once I got properly focussed on looking for the entries in the registry - found them all and temporarily re-named them, Rapture installed as intended. I then ran Sonar X2 as administrator - scanned the VST plugins - entered the registration code and all is well after returning the registry entries to their original names.
Since it may help others, here are the registry keys which I found on my computer:
HKEY_CURRENT_USER/Software/Cakewalk Music Software/Dimension Pro
HKEY_CURRENT_USER/Software/Cakewalk Music Software/dp
HKEY_LOCAL_MACHINE/SOFTWARE/Cakewalk Music Software/dp
HKEY_LOCAL_MACHINE/SOFTWARE/Cakewalk Music Software/dple
HKEY/LOCAL_MACHINE/SOFTWARE/Wow6432Node/Cakewalk Music Software/Dimension Pro
HKEY/LOCAL_MACHINE/SOFTWARE/Wow6432Node/Cakewalk Music Software/dp
HKEY/LOCAL_MACHINE/SOFTWARE/Wow6432Node/Cakewalk Music Software/dple
It seems that missing just one of these entries will trip the installation process up. I simply added the word "backup" to each name when I did the initial re-naming.