@spursboy201568 Smashed screens are not covered by any warranty.
You could return it to giffgaff for a repair, but this will probably be one of the most expensive options and will mean you are without the phone for a period of time. With giffgaff, they will give you a quote on receipt of the phone and if you do not want them to do the job, they will charge you £15 to return the phone!!
I would look to take the phone into a local mobile phone shop and get a quote from them. Many will do a repair on the same day and you will be available to reset any passcodes etc.
What phone is it? Many newer phones have very expensive screen repairs.