You need to show us the original Notice to Keeper (NtK) as that is the only document they can rely on to pursue anyone. You can safely ignore all debt collectors, including Empire and DRP. They are powerless to do anything except to try and intimidate the low-hanging fruit on the gullible tree to pay out of ignorance and fear.
THis almost certainly was not a windscreen Notice to Driver (NtD). It will have been issued as a postal NtK.
If there is even a remote possibility that Horizon hold more than one possible address for you, then you need to immediately email their DPO with a Data Rectification Notice (DRN) the instructs them to update their records with your current address for service and to erase any other addresses they may hold. The highlighted words are there for a reason, so use them.
As this was for an alleged contravention almost a year ago, it is highly likely that their NtK was not PoFA compliant at that time and as long as the driver is not identified, they cannot pursue you as theKeeper. Until you show us the original NtK, we cannot confirm that.
