Just updated the Android SDK using the B4A SDK Manager & now I get the above error in a pop-up when B4A starts certain projects & compilation fails with the same error.
I've searched for packages containing "annotation" & there are 3 of them & they are all installed on my development machine.
Thanks Erel - that seems to have done the trick. I did get as far as downloading auto-value-annotations-1.6.3.jar, but I wasn't sure where to put it & I didn't have the associated .pom or maven files.