We report muon Knight shift measurements on single crystals of Sr 1 4 Cu 2 4 O 4 1 and Ca 9 La 5 Cu 2 4 O 4 1 to determine the muon stopping site in the isostructural compounds of the (Sr,Ca,La) 1 4 Cu 2 4 O 4 1 series. A site close to the ladder oxygen (0, 0.29, 0) is found. In addition, the magnetic correlations in the system Sr 1 4 Cu 2 4 O 4 1 have been studied by zero field μ + SR between room temperature and 2K. Quasi-static muon spin relaxation is found below 10K, whereas thermodynamic and neutron scattering reveal a singlet ground state.