You would COPY (not move) the photos using your operating system to the desired location. Then, following the instructions in Part 2 of this document, use the Update Folder Location command to link Lightroom to the new location of the photos. Once everything is working properly, you can delete the original photos on the old drive.
You want to be sure you have backups of your catalog file and all photos before you do this.
Side issue: there's really no benefit to putting photos on an SSD, this is very expensive real estate for your photos, and you will never notice the speed improvement, the difference between photos on an SSD and photos on a traditional HD is so small that it is effectively zero. See Will an SSD Improve Adobe Lightroom Performance? | Computer Darkroom
Thanks for your swift reply. I'm going to follow your instructions and will report when it's done and (fingers crossed) all is OK. Apologies for such a basic question, but it's the first time this situation has happened and I'll be sure it's the last!
All the best,
You could also try doing the moving of those files from inside LR. But it has been my experience that it doesn't work as expected all the time. Some times it works fine and other times it doesn't. So I myself use the file manager for my OS to do things like you want to do then reconnect the images in LR as per the document linked to.
This is now all sorted - after much arduous re-linking and fixing. Many thanks to those who replied