Success!!!
Traffic was not so bad tonight so I got to My Wifes workplace with a bit of time before she was due to finish.
Following Mike Noc's experience of the benefits of "letting it hang" i decided to give it a go.
I also took my "Thor" hammer in case it was called for.
I had a section of sleeper that i proposed to put under the rear jacking point and leave the car overnight.
As I was raising the car with my trolley jack on another section of sleeper under the central rear jacking point the rear wheel started lifting straight away on the affected side. When I got to the point when I had about another 1/4" to go before i could slide the piece of sleeper under the jacking point the car started to very slowly start to topple off the jack to that side.
Nothing dramatic, but the jack at the back went over.
As i got ready for another attempt I noticed that it was not so low on that side. Things were happening.
A couple more attempts like the first and things were looking good.
I then was able to undo the lower shock bolt without any worry as things were pretty much normal again.
As i was getting the shock off my Wife finished work so i finished taking it off so i could swap the bits over wnd will fit replacement tomorrow.
If anyone finds themselves in the same situation i would suggest that they try jacking the car up and dropping itdown quickly a few times or blocking it up and leaving to "hang" for a bit to see if a bit to see if that does the trick.
Many thanks to all the suggestions.