Left: theoretical line ratios for the [Cii] 158, [Nii] 122 and 205 μm transitions from diffuse ionised gas, plotted as a function of the hydrogen density. We interpolate the ionised gas density from the observed [Nii] 122/[Nii] 205 ratio (blue circles following the black dashed line), which is subsequently used to determine the theoretical [Cii] 158/[Nii] 205 emission for comparison with the observed [Cii]158/[Nii]205 emission (open blue circles), and hence calculate the fraction of [Cii] emission attributed to PDRs. The synthetic [Nii] 122 / [Nii] 205 line ratios (occupying the grey shaded region) estimated via a correlation with the MIPS 24 μm emission (see the right hand panel) are used to interpolate the ionised gas density to compare with the estimated [Cii] 158/[Nii] 205 ratio (open grey squares). Right: correlation between the MIPS 24 μm emission and the [Nii] 205 μm emission. The best linear fit including 1σ limits are represented by the solid and dashed red lines, respectively. In both panels, each data point represents one pixel.