OK got it sorted, removed the blind from the door and took out the inner spring (used a pair of pliers on the end of the tube and screwed the rod and spring out) the reason it was going slack is that one end of the spring has to locate in the plastic end cap that i removed to get at the spring, with 2 pairs of hands the end of the spring was held in position to locate with the plastic end cap which was then tapped into place, roll blind up, give it a couple of turns on the spring and it was sorted.