Hello and thanks for using Just Answer.
I am Al and am happy to assist you with your enquiry.
It would not be normal or reasonable for you to request an annual fee, to be honest. As the land is landlocked, it is pretty worthless to any Buyer, without a suitable right of way being granted to them over your retained land.
The usual scenario, therefore, would be for you to grant the right of way, subject to the Buyer contributing a fair proportion towards the upkeep of the right of way (ie as and when any maintenance/ongoing electric charges for the gate, the Buyer will pay his due share).
You can always ask for an annual fee on top of any maintenance costs, but I doubt very much that any Buyer's Solicitor will advise their client to proceed on that basis.
Sorry this is not the answer you were looking for, but am giving my honest opinion.