Rob, if you can't get access with a small angle grinder, dremel or nut splitter then you will just have to use a good sharp cold chisel.
In the past I have managed with a small nut splitter but you will have to make room by moving the looms slightly to the side.
You could also try jacking the car up so that the suspension is hanging which will put a bit of load on the nut which may just be enough to rattle the nut off with an impact gun.