The 2 links for the guides above give a good idea.
But to keep it short and brief
Correct method - torsion bars completely removed from the car, TRAILING ARM STAYS ON THE BEAM AND IS NOT DISTURBED, height measured, torsion bars reinserted.
Wrong method - Trailing arm smacked off the rear beam (torsion bar remains in on trailing arm side) arm rotated up and arm smacked back on again.
If insurance company is insisting on needing an engineers report which i assume will have to come from an MOT type garage make sure that the way that they do it is correct. I expect theyll charge a fortune though knowing that insurance company are demanding that the car goes back to standard height and that without the engineers report you'll lose insurance.