The usual suspect is a small metal/rubber/metal bush in the lower eye of the Activa Ram. I took a look today and found the bush was indeed at fault but not in the expected way. It had been replaced with a roughly turned nylon bush
More evidence, as if it were needed, that my Activa had been the subject of bodging in the past 
These bushes are not exactly readily available new, being a very specific Activa Component, so I had a dilemma. Either put up with the rattle or try to mitigate the problem whilst I ordered and waited for a genuine part.
In the event I found a piece of hard nylon which was once part of a printer platen. I used it to turn up a new bush in the lathe and offered it up. It fitted fine but it offered no lateral location for the Ram eye in its yoke so I turned up some nylon washers to act as side-cheeks.
Success
my rear is now nice and quiet and the car is a lot more pleasant to drive. There is no telling how long it will last but I'll leave it until it fails. I've no idea of the specification of the material I used to make the bush and there may be a better material I could use. Any suggestions?
If anyone would like one of these bushes, I could easily make one and incorporate a couple of improvements over my initial prototype and possibly using a better material.
I'll report back on how this bush is lasting in due course.