Here's how I fixed it (if you watch the video link above it makes full sense)
1. You will need a long handled screwdriver, small “electrical” type screwdriver, hammer and a pair of pliers.
2. Ok, I assume you have taken the parcel shelf out of the car and is now lying in front of you on the floor??
3. At each end you will see the end caps that holds the roller in. Using the long handled screwdriver push the head into the cap (the one without the finger button that allows the shelf to be dismantled) and with the hammer gentle bang the top of the screwdriver thus pushing the end cap off, set to one side and watch for a spring that may ping out, put to one side.
4. The other side, using the smaller screwdriver “Ping” off the thumb button, it is held in by two pressure clips – I found it easier using the longer handled driver to push up from underneath, and as before once removed tap off the end cap, again watch for the spring
5. You can now slide the roller out from left to right (you will note the left hand – as the though your looking at it from the boot) has a riveted strap, the other end allows free movement out.
6. Now that you have the roller out from the casing this is where the fiddly bit comes to play!
7. Firstly, roll up the roller nice and tightly, then rethread back into the casing as you took it out
8. With the left hand side now back in the casing, ( I found this easier across my knee) put the whole casing in a position so you can get both hands to the end that DOESN’T have the thumb clip (i.e. the left hand side). You will notice that in the end there is a black plug with three notches inside, with your pliers grabs the external and internal plastic and pull, now this can either bring two parts up or the one inner.
9. The end plug has an inner black plastic insert, within this there is another plastic insert; this is attached to a spring inside the tube. It is this inner part that you need to work on, so you may have to either move the inner out of the outer casing (you will see what I mean when you do this than what is described) or if it has come apart already push the outer back into the tube.
10. Now you should have the smaller inner plastic which is attached to the spring, now with your hand twist the black plastic and spring in an anti-clockwise manner until the resistance is strong, now carefully insert the inner back into the outer plastic cap – YOU WILL NEED TO WIGGLE as there is NOTCH on the inner part that locates with inside the outer plastic, when you find the notch this stops the inner parts spinning back on the spring.
11. Once balanced, then get the outer cover, place the spring inside the outer case protruding prong (if you look inside this prong slides into the inner part of the roller)
12. Gently, GENTLY, push the end over the exposed roller, align the two parts (you can see this when you look at the roller form the side) and give a quick push so the inner connects home to the outer casing prong
13. If you hear the spring go boing you have to start again, it is trial and error (took me at least 10 times!)
14. Now, place the other end cap on the other end, ensuring the spring again is in the prong, push on and then reattach the thumb button. Hey presto this should work!