I think you might find that you won't be able to break out the cost of the fence separately. Maybe you don't want to raise this as an issue either... I could be wrong but isn't electricity for commercial (&farming) purposes billed differently? Could lead to ESB looking for more money and pursuing either you or the farmer, with all the subsequent buck-passing about who's liable...
If I were you, I would be kicking myself that I let this informal situation go on so long - it should have been resolved years ago. If you accepted the €60 then the farmer might be able to say that you have accepted this as the payment should you go a legal route.
I would approach the farmer now, clearly and directly, and say that you want it sorted within (for example) a month, and then take action to get the fence switched off after that.