Apart from this 'prerequisite' being VERY confusing and a NON-Standard way of using links it seems to me that you're wrong.
I've got £10 credit listed and still the link do not work.
Why make it so difficult to link my old nr?????
We are aware of this issue and are working to fix it as we speak. I am very sorry for the inconvenience.
As an alternative, simply email your giffgaff username, current giffgaff mobile number, and your PAC code to email@example.com
With the subject stating you'd like to transfer your number.