What phone and browser you using ?
Try a different browser.Download Firefox here
Try logging out from giffgaff and back in this some times resolves this
Have you tried using the giffgaff app ?
Try clearing App cache for the browser
For Andriod device
space or just set an app back to default (like you've just installed it), here's how:
The new app info screen on Android 6.0 Marshmallow.
Screenshot by Nicole Cozma/CNET
Step 1: Head to the Settings menu. This can be done by tapping the cog icon in your notification shade.
Step 2: Find Apps (or Applications, depending on your device) in the menu, then locate the app that you want to clear the cache or data for.
Step 3: Tap on Storage and the buttons for clearing the cache and app data will become available (pictured above).
If you are using an iphone
Delete the Safari cache on iPhone or iPad
Start by clearing out the caches. Note that this will log you out of any websites you've signed into. Follow these steps to clear out the Safari cache on your iPhone:
Open the Settings app, and scroll down to the fifth group of options (with Mail at the top). Tap Safari.
Scroll down again and tap 'Clear History and Website Data'.
Tap 'Clear History and Data'
Hope this helps