Ron,
Thanks again for your response. I think I may have wasted your time as I now see from the frontend of the mapped domain that the uploaded image does indeed show the URL of the mapped domain correctly.
However, it is in the backend Dashboard that the file URL shows the subdomain and not the mapped domain URL. This is not a problem! I could have sworn that when visiting the page source of the mapped domain, that the image had shown the wrong URL , but it doesn't now that I checked again today.
I do see though another plugin causing the non mapped domain to show but this isn't your problem :) Thanks again for taking a look at this and again sorry that I was mistaken. It appears the issue only exists on the backend which I don't care about and im sure thats normal. Have a good rest of your Sunday!