$\lambda\lambda 6731/6717$ ratio, and we also seek to minimise the L1-norm for the fit. That is to say that we measure the modulus of the mean logarithmic difference in flux (relative to H$\beta$) between the model and the observations *viz.*; $${\rm L1} =\frac{1}{m}{\displaystyle\sum_{n=1}^{m}} \left | \log \left[ \frac{F_n({\rm model})} {F_n({\rm obs.)}} \right]  \right |. \label{L1}$$

This procedure weights fainter lines equally with stronger lines, and is therefore more sensitive to the values of the input parameters. We simultaneously fit to those emission lines which are most sensitive to the controlling parameters of shock velocity and ram pressure; effectively HeI, HeII, \[OI\], \[OII\] and \[OIII\] as well as the \[NI\], \[NII\], and \[SII\] lines. In addition, we estimate the gas-phase heavy element abundances using the bright \[FeII\] and \[FeIII\] lines, \[FeV\] 4227Å \[FeVII\] 6087Å \[NiII\] 7378Å MgI\] 4561Å, CaII 3933Å and \[CaII\] 7291Å lines. We assume that the more excited species of Fe are depleted in the gas phase by the same amount as FeIII. For the reason given above we do not attempt to fit either the \[FeX\] 6374Å or the \[FeXIV\] 5303 Å lines, given that they arise in a separate phase of the ISM. Other elements such as 