I'd be grateful for any views on whether current rules , particularly in possessions, are practicable and / or unecessarily complex.I'd also be grateful for advice of any published research on this topic

As part of our paper for the Rail Network conference (which was postponed) I did some analysis of RAIB accident reports released in 2009/10. This showed that about a third of all accidents investigated by RAIB had Rule Book compliance issues as a causal factor. I particular, two issues relating to possessions appear to be dominant; moving (or rather not moving) to a place of safety when a train approaches and excessive speed in long possessions (or worksites). See RAIB reports 30/2009, 29/2009, 24/2009, 19/2009.

Thanks John, I'll look into that. You might find the article that I've just had an article published on this subject in Modern Railways of interest