You can order a replacement SIM along with reporting lost and stolen by login onto your giffgaff account from here: http://giffgaff.com/support/lost
On completing the replacement form above you'll be given the choice to either "Bar SIM only" or "Bar SIM and Handset" so please ensure you only click on the "Right Option".
The replacement SIM will take up to 1-3 working days to arrive. Once arrived then all you need to do is login onto the account and activate it from here: https://giffgaff.com/activate-sim
The activated replacement SIM will have the same number, credit and goodybag validity date will remain the same.