Simply put an axle stand under the jacking point.
You really need a trolley jack for the job. With the rear of the car supported as described and the wheel removed, place the trolley jack under the rear hub, disconnect the shock absorber and lower the hub until you can remove the spring. You might need to use a pry bar to move the arm sufficiently to get the spring out.
Fitting is a reverse of removal, make sure you tighten all the fixings properly!.
"Motoring Which?" showed a picture in I think one of their 1963 reports of a Renault R8 that had distorted around the jacking point when they were testing it. This on a new car, rust hadn't got a hold of it in its first year of use.
just a bit of feed back
although uneasy about using the jacking point to support the car I used it and it was ok
I had to use spring compressors on the new spring to get it in place (not as strong as I used to be, so could not force down the spring base plate far enough
also had trouble getting the locking wheel nut off, as it looks like the head is a bit cut up, so I'm going to replace all the locking wheels with a different type