So I took it apart and nothing seems excessively worn or out of place. On reassembly with a little grease it is now a consistent (100%) problem!
A little video
MOV_2234 by jeznewsome, on FlickrI have tried the spring in a number of orientations with no change in behaviour.
I assume that there is something wrong with the geometry of the stand? It is that the 'stop' is not at the right angle i.e. the stand doesn't rotate far enough forward to go 'over centre'
Anyone able to provide a photo of a working stand to show the angles etc? I'd like a little guidance before I start attacking the area with a file
TIA

