It sounds right to me, if you get 4.1p /kWh and pay 5p / kWh then it will be 9p /10kWh.
You are on a good rate with Octopus, enjoy it whilst you can! At some point it might be worth installing a battery to store your solar or cheap overnight rate, especially for Nov-Feb when your solar pv will not give you as much.
VW and other companies are developing car-2-home for their EV’s, so that you can utilise your car battery to power your home and also sell back to the grid at peak times but re-charge at off-peak, so that might be an option for your next car.