Thanks. I installed liblensfun-bin, ran the command above without privilege, and got
a success message updating .local/share/lensfun/updates/version_1
and now there are 6 Samyang lenses shown rather than 3 before. But not the 35mm !
I should have said, I’m on Ub. 18.04.4 and dt 3.1.0~git125.05f1bf829 which I built myself. Just had a look at the log I took at the time and can’t see any odd messages in connection with lensfun.