@darvell the quickest way is pop into likes of pound land Tesco asda Wilkinson b&m Sainsbury or Mc colls buy a new 3in1 SIM for around 99p then login to your account and follow this guide to swap current number to new SIM card sim swap http://community.giffgaff.com/t5/giffgaff-Top-Tips/How-to-use-SIM-Swap/td-p/6885194 please remember sim swaps cannot be done between 9.30pm and 4.30am
also remember to restart phone after approximately 30mins
If you haven't already Please follow this link http://giffgaff.com/support/lost to report the SIM and phone lost/stolen to prevent anybody from using it. Once reported giffgaff will send out a replacement SIM first class. It will be delivered to your registered address found under your details page here http://www.giffgaff.com/profile/details
The replacement SIM will have still have your existing number & any active allowances e.g. goody-bag or/and Airtime credit. It should take no more than 3 working days for the replacement SIM to arrive.you'll need to activate it online when it arrives here http://giffgaff.com/activate-sim .