Posted: 20/10/2013 at 09:14
It's likely to be a deficiency, Matt. I'm not sure that it's magnesium. Magnesium deficiency usually manifests in a two-tone leaf. Have you fed the plant with a dedicated citrus fertiliser? If not, give it a feed - dose according to the instructions - and see what happens.
One feed should be enough. It's late in the season and you don't want to encourage too much new growth with winter on the way.