Context. Shock modelling predicts an electron density enhancement within the magnetic precursor of C-shocks. Previous observations of SiO, (HCO+)-C-13, (HNC)-C-13 and (HCN)-C-13 toward the young L1448-mm outflow showed an over-excitation of the ion fluid that was attributed to an electron density enhancement in the precursor. Aims. We re-visit this interpretation and test if it still holds when we consider different source morphologies and kinetic temperatures for the observed molecules. To do this, we use updated collisional coefficients of (HNC)-C-13 and SiO with electrons in our excitation model. We also aim to give some insight on the spatial extent of the electron density enhancement around L1448-mm. Methods. We estimate the opacities of (HCO+)-C-13 and (HNC)-C-13 by observing the J = 3 -> 2 lines of rarer isotopologues. To model the excitation of the molecules, we use the large velocity gradient (LVG) approximation with updated collisional coefficients to i) re-analyse the observations toward the positions where the over-excitation of (HCO+)-C-13 has previously been observed [i.e. toward L1448-mm at offsets (0, 0) and (0, -10)]; and ii) to investigate if the electron density enhancement is still required for the cases of extended and compact emission, and for kinetic temperatures of up to 400 K. We also report several lines of SiO, (HNC)-C-13 and (HCO+)-C-13 toward new positions around this outflow, to investigate the spatial extent of the over-excitation of the ions in L1448-mm. Results. From the isotopologue observations, we find that the emission of (HCO+)-C-13 and (HNC)-C-13 from the precursor is optically thin if this emission is extended. Using the new collisional coefficients, an electron density enhancement is still needed to explain the excitation of (HCO+)-C-13 for extended emission and for gas temperatures of less than or similar to 400 K toward L1448-mm (0, -10), and possibly also toward L1448-mm (0, 0). For compact emission the data cannot be fitted. We do not find any evidence for the over-excitation of the ion fluid toward the newly observed positions around L1448-mm. Conclusions. The observed line emission of SiO, (HCO+)-C-13 and (HNC)-C-13 toward L1448-mm (0, 0) and (0, -10) is consistent with an electron density enhancement in the precursor component, if this emission is spatially extended. This is also true for the case of high gas temperatures (less than or similar to 400 K) toward the (0, -10) offset. The electron density enhancement seems to be restricted to the southern, redshifted lobe of the L1448-mm outflow. Interferometric images of the line emission of these molecules are needed to confirm the spatial extent of the over-excitation of the ions and thus, of the electron density enhancement in the magnetic precursor of L1448-mm.