You need to get the torx screw out to lower it properly, also heat around that area wont do the already fragile beam bearings any good,
you can lower it the "Pikey/Notch" way without removing that screw , you will have to undo the bolt on the other end of the torsion bar, and smash the back of the trailing arm with a sledge hammer, but this will only allow, adjustments of, 20,40&80mm IIRC so do that side first then match the otherside,
But as said doing it this way isnt ideal as your only pulling one spline out one end instead of pulling both splines giving total free adjustment
you can lower it the "Pikey/Notch" way without removing that screw , you will have to undo the bolt on the other end of the torsion bar, and smash the back of the trailing arm with a sledge hammer, but this will only allow, adjustments of, 20,40&80mm IIRC so do that side first then match the otherside,
But as said doing it this way isnt ideal as your only pulling one spline out one end instead of pulling both splines giving total free adjustment