Detailed measurements are presented for four independent optical centres with zero-phonon lines at 951.16, 952.98, 953.96 and 956.91 meV which are observed in 2 MeV electron-irradiated carbon-doped silicon. Uniaxial stress perturbations and carbon and silicon isotope effects are all shown to be very similar for these four centres and for the well known 969.45 meV centre, implying that all these centres are minor modifications of each other. A re-examination of the 969.45 meV vibronic sidebands observed in luminescence and absorption.