Hi there @daveytoonmarshall
What device are you using please ? .
When you buy a 4G goodybag for the very first time , it may take up to 24 hrs to activate your 4G service,but you'll still receive 3G until your 4G service has activated.
Also go to your device " settings " and Select " More networks " , then select " mobile networks " and then select " network mode " and select LTE / WCDMA /GCSM settings to Automatic.
Please can you then turn your phone OFF and ON again,over the next few hours or so and see if that helps.
Please also check you're in a 4G area here http://community.giffgaff.com/t5/Using-giffgaff/Guide-to-4G/ta-p/12134248