Guys,
Thanks for the help/advice - it was actually very simple in the end, and I have the sound back and am able to continue working on the project - such a relief!!
All I did was get the original set of 4 Discs that came when I bought Sonar 8.5, "Additional Content" was on Disc 4 of 4, so I installed the sound libraries content (but not the other demo projects stuff) to the correct file path, (e.g. C:\Program Files\Cakewalk\Dimension Pro), and when I reloaded the Project DiPro automatically picked up the right multisample with out me even having to search for it!
I must admit to being a little surprised that in 'upgrading' to X1 it didn't include this previously available content?! But I will now know for future clean installs. I'm about to install X2 so hopefully this won't happen again!
Hope this of use to anyone else who has lost vital sounds for their projects after re-installing from 8.5 to X1/X2!
Many thanks again!