Colour-colour diagram of the inner lobes, showing the SPC of the plasma inside the selected regions. Two different spectral ageing model curves are overlaid onto the point. The blue dots are the spectral index values that we measured in the 11 regions shown in Fig. 6. The dashed grey line represents the bisector line where = . The red line represents the JP single-injection evolution model with α_inj ∼ 0.6, while the green line is the same model with α_inj ∼ 0.7.