Summary is self explanitory.
Having the myro-cpp-config script makes this not much of an issue.
Turns out this was because the path in the CMakeLists.txt file was absolute (using the install prefix variable) rather than just being relative like "lib/pkgconfig" so would still install fine for a "make install" but wasn't picked up by CPack.
Bug watches keep track of this bug in other bug trackers.
Having the myro-cpp-config script makes this not much of an issue.