So, just an update on this. Before christmas I had finished the wiring on the trailer and gathered the parts needed for the car side. Over the last few days, I've finished off the in-car wiring; isolator, and 175A anderson plug for the rear of the car. Oh, and I did buy a roller at a boat shop and made up a mount to use as a roller for the wire over the centre bar - no pics of that though.
Cables are run in conduit cable tied up out of the way under the car - was a pain feeding the two wires into the 25mm conduit out in the hot sun let me tell you!
To finish off, I just need to go back to where the trailer is in storage (50min away) to verify the length of cable needed to make an "extension cord" using two more Anderson plugs, that fits between the Anderson plugs on the trailer A frame and the rear of the car. This will only be connected when I want to use the winch rather than left on all the time.
I had ten metres of conduit and blue/red wire and I reckon after finishing up the "extension cord" i'd have a bit over a metre left over at most, so the initial estimate of 8 metres needed given by you guys was pretty accurate.