Thanks for your replies. The version of Garage Band currently on the App Store is 10.4.8. And as far as I can tell, this version requires macOS 10.12 (Monterey). I do have Garage Band v. 10.3.5 available from a Time Machine backup of my Mojave system - but it doesn't run in Big Sur.
It appears then that the current Big Sur upgrade package doesn't include a new version of Garage Band. Which means that even in my case, where I had probably the final version of Garage Band in the long run of macOS X, I would not be able to run Garage Band.
I have gone looking for releases of Garage Band earlier than the current 10.4.8. It stands to reason that one of these must have been runnable under Big Sur when it first came out. But so far, I don't know what that version was nor whether or not a download is anywhere to be found.
So it looks like if I want to run Garage Band at this point, I have no choice but to upgrade (once again) to OS 12/Monterey. Which I'm reluctant to do given I'm just getting familiar with OS 11, a non-trivial jump from the familiar confines of OS 10 ;-)
If any does know which version of Garage Band will run with Big Sur and where it can be found, I would be grateful.
Thanks