Many iPhone on giffgaff have a problem where when you send messages to some people, your old number appears rather than your new one. There is an easy fix for this.
1) Go to Settings
2) Go to Messages
3) Switch iMessage OFF
4) Once it's switched off, turn it back ON again.
5) Wait for it to finish activating iMessage
6) Once it's done, reboot your phone.
Your messages should now display the right number.
Short link to this guide for new members: http://bit.ly/iphonemessagesoldnumber