John S is absolutely right. I've just repaired my Velux blind (no tension in spring) by following his instructions. To go into a little more detail, if you've got the blind tube out, try turning the rectangular bit of metail clockwise with a pair of pliers - if it doesn't tension up then the spring has come unscrewed inside. If, using the pliers, you pull on the rectangular bit of metal as though you're trying to pull it out of the roll, the whole mechanism will slide out - you may have to jiggle it to get the final end piece out. Once it's out, simply wind the spring round the black plastic screw thread like you're tightening a screw. Once you've done this, you can reinsert it, noting the dog-tooth gap in the black plastic end which needs to be matched to the insert in the blind tube. Before re-installing, you need to tension the spring - I didn't use a special tool, I simply rolled up the blind fully, then let it out by about 2" then used the corresponding black plastic end fitting to wind up the tension (clockwise) - maybe 20 turns. Note that the rectangular bit of metal is slightly concave so only fits one way into the black plastic. Then fit it all back together!
Duncan B, April 2016