Measurements are reported on the visible and near ultraviolet spectra of type Ia diamonds after room-temperature electron irradiation. The temperature-dependence of the GR1 doublet has been determined in diamonds of low nitrogen content, between 9 and 130 K. It is shown that these data enable limitations to be placed on the degeneracies, and hence irreducible representations, of the electronic states of the GR1 centre. With increasing nitrogen content the GR1 doublet is found to become complicated by the growth of extrinsic structure. This is shown to result from some GR1 centres having migrated to specific sites close to the nitrogen, so that their symmetry is lowered, lifting the orbital degeneracies. ND1 centres are found to be produced in proportion to the GR1 centres, regardless of the nitrogen content, once the diamonds are recognizably type Ia. Again, in diamonds of high nitrogen content, some ND1 centres are severely perturbed by the nitrogen, resulting in the ND1 zero phonon line's long tails. Annealing rapidly destroys these perturbed centres as the nitrogen captures them. The ND1 line width is then only slightly dependent on the nitrogen content, in agreement with the stress coupling parameters of the ND1 line. Attention is drawn to the 503.6 nm band, which has probably been confused in the past with the H3 line at 503.2 nm. The 503.6 nm band may be formed in any diamond; in type Ia diamond its strength is found to be proportional to the radiation dose and, approximately, to the nitrogen content. It is rapidly annealed out.