Brilliant instructions, thanks.
I've replaced the cord in my Velux blackout blind without much trouble. I used the whipping and superglue method to join the new piece to the old where it comes out of the sealed capsule at the top and left all the length on until fitting the bottom bracket back onto the window when I could tension the cord appropriately.
Now it works better that ever as I was also able to realign the lugs on the sides of the blind back into the channels. Saved £150 UK which was the store's estimate for repair.
Couldn't have done it without these instructions.