Given that National Grid have carried out substantial repairs to the "new" tunnel, I think it is only right that they should be compensated by a "new" tunnel in the old bores.
Depending on what state the old tunnels are in, it is technically possible to re-line them with either pre-cast units and filling in the void between them and the old lining with grout, or shotcrete the existing tunnel after suitable structural repairs.
Moving the new cables out of the new tunnel isnt a show stopper for re-using the new tunnel for rail purposes - it just means removing the 400kV plant out again, removing the concrete slab and all the other bits NG have put in there. Might want to keep the tunnel lighting though!
I dont see what all the fuss is about really. Its not like there are any firm proposals for a rail line in at least the next 15 years, by which point the "new" NG stuff will be approaching half-way through its design life anyway.