Spoke to Virgin rep on 789 - although voice and text working, data connection on 4G sim will not activate until date of monthly renewal. But can't use original 3G sim as that has now been deactivated! No internet connection for 3 weeks outside of WiFi areas. This needs to be highlighted better when people transfer to 4G sim. Basically keep using original sim until end of monthly contract date then swap over.
The old 3G sim gets deactivated not the 3G service, so like what has been said already, as soon as your refresh kicks in you'll be able to use 4G unless you're in a poor reception area then it'll default to 3G.
Followed your advice - spoke to tech section and they have sorted it. Data connection is now working. Back on 3G for a couple of weeks until renewal date then will transfer to 4G. Thanks to all of you for responding.