that the theoretical models and the observations show a satisfactory overlap.

This said, a few points on these diagrams are worthy of further mention. First P13 is the only region which shows an excess in the line intensity of \[\], which may be indicative of pre-supernova mass-loss enrichment. However, this seems a little unlikey as the region is close to the boundary of the SNR shell. Second, P20 shows extremely strong \[\] emission. This is evidenced by the blue appearance of this region in Figure \[fig3\]. This region will be discussed in more detail below, where we show that it is due to a finite-age shock in which the gas has not had time to become fully radiative. Lastly, P08 shows particularly weak \[\] emission. This region, also discussed below, lies in a complex of Balmer dominated or non-radiative filaments in which a high velocity non-radiative shock is passing through an un-ionised or partially-ionised precursor medium.

 \[\], \[\] and \[\] diagnostics
--------------------------------

As can be seen from Tables \[tableA1\] - \[tableA4\] the \[\] spectrum is very rich. For the purposes of analysis, we have selected the brightest of these lines, \[\]${\lambda 5158}$ and \[\]${\lambda 8617}$. In Figure \[fig8\] we present 