@jonas On checking - I had protobuf-compiler already installed (so why did I install via snap??!!) I removed the snap version and re-ran cmake - make then gave me
ext/libstrawberry-tagreader/CMakeFiles/libstrawberry-tagreader.dir/build.make:73: *** target pattern contains no '%'. Stop.
I removed the build directory contents and then cmake/make ran ok - not sure whether cmake should just have worked with the dependency change but it's now ok.