If your goal is just to install Qsampler, there's a PPA which Rui always
keeps up-to-date with the latest version. The applications and libs are
in separate repos with the same signing key, not sure if you need both
for Qsampler.

Thank you for your reply, Roger. I was sent a personal e-mail advising me to install qttools5-dev-tools. This included /usr/lib/qt5/bin/lupdate. After doing so, the source code compiled and installed without error. As I mentioned, I was missing something obvious. Still, unless you know it, you don't know it....

Tom