It is way too late to make a POPLA appeal. However, I would bet my bottom dollar that Smart Parking do not have a valid contract to issue PCNs at Argal Reservoir car park.
If you have identified the driver by inadvertently admitting it in your appeal, then you have lost a useful defence point.
Argal Reservoir is owned by South West Water and managed under licence by the South West Lakes Trust, as confirmed in the High Court case Brown & Ors v South West Lakes Trust & Ors.
While specific byelaws for Argal Reservoir are not publicly listed, it is common for utility-owned reservoirs to be subject to byelaws under legislation such as the Water Industry Act 1991. These byelaws typically govern activities like fishing, boating, and public access to ensure safety and environmental protection. The South West Lakes Trust enforces rules at Argal Reservoir.
For detailed information on applicable byelaws or regulations at Argal Reservoir, you should contact South West Water or the South West Lakes Trust directly.
If byelaws do apply (likely), then as the Keeper of the vehicle, you could not be held liable. Only the driver can be liable and they have no idea who the driver is unless you blab it to them, inadvertently or otherwise.
For now, all you can do is ignore all debt recovery letters and wait for an LoC which will not come from Smart but a bulk litigator such as DCB Legal.