Hi Alun,
If you don’t have the Alpy calibration unit, you can use the Balmer lines in your A-type spectrum to find the polynomial which maps pixels into Angstroms. You can use that to calibrate your galaxy spectrum independently of the sky background lines and use the sky sodium line as a check that everything is correct. You can then measure the galaxy redshift in km/s using the H-alpha line (delta lambda x 300,000 / lambda) which is always a satisfying thing to do with a small scope in your back garden. Once you have done the background subtraction, you will probably find a small absorption line at the sodium wavelength in your spectrum due to absorption of light from the distant galaxy by sodium in our own galaxy.
Cheers,
David