Hi Alun,
According to the NED database http://ned.ipac.caltech.edu/forms/byname.html NGC 2903 has a redshift velocity of +550 km/s which corresponds to a wavelength shift at H-alpha of +12A. It’s hard to tell exactly from your plot but that looks close to what you are seeing. The NED database also contains spectra of the galaxy which show it has a fairly flat continuum with only a few strong emission lines at H-alpha, H-beta and (probably) [N II] at 6584A, all of which you see faintly. So the stronger emission lines you are seeing such as sodium D at 5896A (which is not redshifted) may be sky background lines. Did you subtract the sky background during processing?
David