After some quick debugging last night, I discovered that whilst refactoring the isDriver check a few weeks ago, I accidentally removed the tonumber() conversion of the vehicleID, this means the check was always returning false, hence the script never saw that the player was in a vehicle.
Quickly adding these checks back in resolved the issue, and the isDriver function is now working as expected.
I have also pushed all of the changes over the last month up to git.