To slacken, you turn the allen key clockwise, and the top nut anti-clockwise. If you intend to re-use the strut, you should actually keep the allen key still (to prevent the shaft from spinning in the shock absorber) and turn the nut counter-clockwise. However, since the allen key hole is broken, you will never be able to fasten the top nut to the published torque accurately, so a replacement shock absorber is required. Remove the shock absorber and top mount assembly and take it to your local garage to get the top nut blasted off with an air impact wrench so that you may retrieve your top mount, washer and bushing. It may be a false economy to replace with secondhand dampers since a pair of Monroes with a 2 year warranty would cost less than £70, including the new top nuts that you will require.
When doing it all up again, remember to set the torque properly. For the top nut, a regular torque wrench will obstruct the allen key hole, requiring crow's foot adapters. Don't forget to use an appropriate conversion formula when using a crow's foot adapter.
Edit: Don't forget to have coil compressors applied before loosening the top nut, or tightening a new one down. There should be no pressure from the spring until the top nut is in place and torqued properly.
Don't worry though, damper top nuts have the best of us beat. I've done this procedure three times now and every time has been a pain in the backside.
