@andym37041 did you top up by minimum ten pounds voucher to activate SIM as shown here and click on activate by voucher http://giffgaff.com/activate. Only after doing this will Sim be given a number to use once completes your number will show here http://giffgaff.com/dashboard/
Hopefully that helps out
You need the voucher to topup as credit or purchase a goodybag at the same time as activating as It will not work,
For full instructions on activation see here:
Hope this helps.
You must redeem the voucher whilst you are registering the SIM card. Without this, the SIM will never activate.