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Doggit
If you have a purlin to hand, then you also have another one across the loft. You can either knock a bit of 3x2 between the 2 purlins to brace them, or you can install a bit of 4x2 across them cut at either end for the angle of the roof, and then put a couple of M10 bolt through both ends. That'd let you run the block and pulley from the brace.