The plot thickens!
I thought the struts and the whole SLS system was self-bleeding which implies that any air anywhere will bleed out.
Is it possible that there is air in my struts? This could be the root cause of the annoying knocking I get from the rear.
The car is a break (240TD) so there is access to the wheel arches above the struts I think - under a rubber cap.
As you mention, the fluid will be under pressure from the weight of the car and the air will rise so do I assume that just loosening the joint should be enough to bleed air?