There's a unique washer under the head of the bolt which has a tab bent up over one flat on the (specially modified) bolt. Have you folded this back down? It can be difficult to see under the accumulated detritus...
I have sprayed the three bolts that hold the radius arm on and left it 25 mins . I then bent locking washers back and cut the wire tie on the other bolt and picked up the 19mm ring spanner and expecting big problems ..... I am pleased to say all three bolts came out very easy and the small strap bolt at the larger end looks like new on the thread part now removed. The shock mounting bolts now need to be removed in order to pull the special bolt out but can not see any problems for this side so far.Ring spanner is the best, and yes it is to clear the "tube" for the coilover bolt. The bolt rarely seize in the thread, it is mainly the radius arm bush that rust to the bolt. Try blue (the lowest grade) LockTite, it will keep moisture out and won't hold too hard. Even if stubborn heating to 100C will make the blue give up it's hold completely.
A large screwdriver giving a few knocks (sideways) on the inside of the cup stuck to the floormounted "cup" will normally see it fall off (not sure if that was what you meant.)I have sprayed the three bolts that hold the radius arm on and left it 25 mins . I then bent locking washers back and cut the wire tie on the other bolt and picked up the 19mm ring spanner and expecting big problems ..... I am pleased to say all three bolts came out very easy and the small strap bolt at the larger end looks like new on the thread part now removed. The shock mounting bolts now need to be removed in order to pull the special bolt out but can not see any problems for this side so far.
The large bush is stuck to the floor and needs a lever etc. Is this best to give the center of the bush a bang with a large screw driver where it joins the floor ? Any other way better ?
I just hope the other side bolts come out just as easy !
The cup in the arm bush gets seized to the upside down 'dome' on the bodywork. As cold chisel between the two (in the gap just above the bush/arm) and a few huge belts with a lump hammer and off it will come. Support the car on the jacking point just to the side and behind the arm, OR jack up under the middle of the rear suspension under the diff on the cover plate and support it there. That way the suspension arm can still move.
When reassembling, do not tighten the small end bush on the suspension arm until the car is back on its wheels and the suspension and the suspension arm is in its normal position at its normal angle, otherwise the bush tends to fail quite quickly as it is under permanent stress.