Thanks, I think I am almost there now.
I have found the page on 123-reg.co.uk to update my dns on there servers.
I have a number of options.
Name - Type - Content
@ - A - 126.96.36.199
ftp - CNAME - mydomain.co.uk
mx0 - 212.xx.xxx.xxx
mx1 - mx0.123-reg.co.uk
www - A - 188.8.131.52
1 - Which of these do I need to change?
2 - Will it take 24-48 hours for the change to take effect?
3- Do I need to change the web forwarding?
4- Once done should dig then point to my public ip?
Sorry for all the questions, and thanks for being such a great help.