Ah sorry about the confusion. I am trying to to calculate the electric field. I understand that I have got a complicated system which is why, as a physicist, I am attempting to model it as simply as possible.
If I propose another example. A high voltage power line on the UK supergrid, 400kV, standing 10m from the ground. If I were to model it as a single infinately long cable what is the electric field strength on the ground?