Final update to help anyone with similar issues. I am pleased to say the fault is now resolved. As originally suspected the cable was damaged somewhere underground between my home and the nearest node. This was replaced and everything is back up and running. The local Virgin engineer was excellent and extremely helpful and overall the response was probably as speedy as I could have expected from a damaged underground cable.
Some useful information for anyone experiencing similar issues:
1. A red flashing light on a 5x Hub means it is disconnected from the Virgin fibre network.
2. This can be due to the cable / connectors being broken in your home or a fault outside your home somewhere in the network.
3. If you are using the new 5X hub and XGS-PON fibre connection and need to call Virgin to report a fault, mention that you are using a 5x Hub first so that the call can be passed to the 2nd tier technicians.
4. New fibre networks recently installed to an area may have been installed through a partnership / outsourcing between Virgin and another business, in my case Avonline. Avonline built the local fibre network for Virgin and have a guarantee to repair faults arising from the installation. A virgin engineer therefore cannot attempt to repair a network fault. Instead a ticket needs to be raised with Avonline. In my case, Avonline resolved the ticket within a few working days and their team were very friendly and helpful during their visit.
I hope this helps anyone with a flashing red light on their 5x Hub!