I have written this small guide for those giffgaffers who want to manually set up their internet on their Samsung Galaxy S2. Should work on other Android (Version 2.3) phones as well, just slight differences in APN layouts.
1) Head into Wireless and Network Settings
2) Go to Mobile Network Settings
3) Go to Access Point Names to create new giffgaff APN
4) Create New APN using the info shown, password = password
5) New APN should show up after saving
7) Select Use packet data to activate data newtwork
8) Enjoy your unlimited data :-)