Hi, for card related payment issues, pls see here:
Glad it's sorted out for you.
I have been having exactly the same issues. I Hvae tried Chrome, safari and Firefox on OS X and Linux machine. I have tried clearing coockies and caches and even using incognito mode.
I spoke to one of my credit card providers and they said the decline was issues from them because of the "test transaction of 0.01" that is used to verify you have entered a vliad credit card.
I have tested a few things and it verification is definitatly failing on the final GiffGaff payment processing after the card provider has returned a positive result from the SecureCode system (thanks to my MasteRcard provider for looking in to this).
Hopefully someone at GiffGaff will get a chance to work out what the issue is soon.