Login to your account at 123-reg. Select the domain you wish to transfer and click 'Manage'.
Scroll down to Domain Transfers area and click 'Request Auth Code'
After a short wait you will get a code like this
Go to Nutty About Hosting and insert the domain name and click 'Transfer'
You will be given the number of years you wish to add to the current renewal date.
Add the Auth Code you got from 123-reg
Once you pay for the domain you will receive a comfirmation email to verify that you wish to transfer the domain. You MUST click this link otherwise the transfer will not take place.