I only have one instance of Mercury running. You are right, I did setup two MX records, smtp.aviandirectory.uk and mail.aviandirectory.uk because I was not sure if I needed both.
I just deleted the MX record mail.aviandirectory.uk. Will post a screenshot of the GoDaddy settings, I am no pro, but I am trying the best I can.
I will make the changes in Mercury and post back the results