Originally I had Sonar Artist, and was very pleased when Sonar was rescued, and I now have CbB which gives me extra features if I need them. I notice that when using CbB I can still access all the instruments that came with Sonar. I think I can still download these from Cakewalk Command Centre, but I suppose that facility will disappear at some point in the not-too-distant future. I note they are not accessible via Bandlab Assistant. What's the best way to back these instruments up so that I can reinstall them in the future if I had a major computer problem?

Open up Cakewalk Command Centre and click on the cog at the top right.  This will open the settings page.  Head to the “paths” tab.  On the “paths” page, you should see a setting "Download Path".  Open that path in explorer.  This location should contain everything that you have downloaded via Cakewalk Command Centre.

Simply copy the contents of this folder to an alternative location (or include this folder in a regular backup).

promidi
Open up Cakewalk Command Centre and click on the cog at the top right.  This will open the settings page.  Head to the “paths” tab.  On the “paths” page, you should see a setting "Download Path".  Open that path in explorer.  This location should contain everything that you have downloaded via Cakewalk Command Centre.

Simply copy the contents of this folder to an alternative location (or include this folder in a regular backup).

Thank you so much for your help, and your instructions which were really clear.

Although you can keep copies of the installers, it is not clear to me that you can save any activation token for versions that require activation. You can download an activation file using the offline activation procedure, but if you need to move to another machine in the future, that file will not work on another machine. So unless BandLab is going to keep serving up activation files, at least some of the Cakewalk products will eventually go bye-bye. 

BandLab has made it pretty clear they're not going to leave anyone high & dry with Sonar downloads/activations, so until the migration of previous cakewalk products / accounts to BandLab has been completed, I think it's safe to assume the CCC activation will continue to operate until then.
 

If you're worried about having what you need to activate, use the PrntScr button on your keyboard to take a screenshot of your account.  Open a graphics program like https://www.irfanview.com/ and Cntrl +V will paste the screenshot.  Then save the graphic image with the rest of the files.