There could be a few circumstances that are preventing you from receiving 4G coverage. Check what handset you are using, if it is an older handset you may need to check if it is 4G compatible or whether there is a setting you need to change to enable 4G. Are you currently on a 4G tariff? You would also need to be on one of these tariffs to receive 4G.
Thanks for taking the time to reply. I have checked all these things and I should be able to get 4G. After contacting Virgin they offered to send a replacement sim. Sadly, on installing the new sim today my iPhone still says I’m only getting 3G even though 4G is enabled on my phone.