Thanks for the tip everyone!
I went to the hardware shop and bought some curtain wire (spiral spring encased in a vinyl outer) but it was too flexible for the long run up the C pillar and it bent before it could get around the corner. I also bought some 3mm braided wire (for sun shades etc) but it was also too thin and bent too.
What did work for me was going to a bike shop and buying 1.5m of Gear Cable Outer which is flexible but does not bend as easily, that worked a treat. I'd recommend buying 2m if you can as 1.5m was a little short for that run up the C pillar and only just made it.
I did find a route - further back towards the rear door is a large oval shaped opening which connects to the C pillar. And on the opposite side of the van, there is another oval shaped opening. These are pretty sharp so it's best to cover them with some U shaped channel edge protector. I found some in my local hardware shop.
I tried both ends - from the top down did not work as I couldn't guide the cable could in to the C pillar cavity, i tried many times and it did not work. Working from the bottom and pushing upwards did work - i could add a little rotation on the wire to guide it in to the right direction on the T junction in the C pillar.
The next issue was the cables got snagged on one of the corners and I needed to somehow dislodge them. I created a tool out of some 3mm solid wire, similar to a coathanger but all straight, and made a U shaped hook on the front which would follow the stuck cable up and I could apply some pressure to push it upwards and dislodge it. It took a few tries, but it worked well.
