There's huge blue signs advising drivers to pay the Blackwall tunnel tolls. Maybe suggest that whoever was driving your car pay the fine? I don't see any other way round it.
As another respected member said on here recently regarding speeding and camera locations, Something along the lines of use the huge picture built into your car called the windscreen and stop looking at the small screen of the satnav. The coucil helpfully puts signs up to warn you of things on the road.
Maybe the Dartford authority will refund your fee paid there, I don't know, I have an account with them so don't need to worry. Similarly I have a TfL account which covers all things like Blackwall Tunnel, ULEZ, C zone etc. Maybe phone TfL to see if they'll drop the fine, but don'y hold out much hope.
