All finished, my old cord was 4m long, so I marked the new cord at that, threaded it through with a piece of flexible plastic, I drilled a 4.5 mm hole in it and poked the new cord through. It worked a treat and is now fitting lovely again.
Thanks for the info on here, always helpful.
I'll try and put some photos on...if I can remember how

Did ours today with 4mm cord, I used a 200mm length of thick strimmer cord taped to the end of the cord to feed through the canvas sleeves at the corners.I have left the old cord in place as a sort of belt n braces solution , so if one fails the other is still there. I bought 10meteres which cost 12.99.I have 5.5 meteres left. Took me 20minutes in all.

The easiest way is to tie the new cord to the old one and pull the old one round the van and the new cord will follow. Simples.