many thanks to all the contributors to my questions, and many thanks Gonzo for the link to the official HAL release and explanation of the trial.
A couple of questions or comments
1 It seems that for many years the original routes were not that strictly followed with aircraft being given different headings on both MID and SAM SIDS as they climbed out of LHR. No one seemed very bothered by that noise wise.
2. The new trial does bring the MID and SAM routes much closer together than the original routes which saw MID departures turning south for Midhurst quite soon after initial climb and SAM departures making it to just south of Reading before heading for SAMPTON. However as per 1, these were not set in stone and the actual picture was more scattered.
3. I would really appreciate a professional view on this but surely the old method was more resilient since separation could be achieved and maintained by turning aircraft south at differing intervals rather than have them stay in trail on a very narrow track where differences in speed and climb rate would seem to cause delay on take off clearances rather than ameliorate them or 'to make performance more resilient'
4. There is also the comment about more reliable separation from other aircraft in the London TMA such as LHR inbounds and Gatwick outbounds.
I don't quite see that either since the minimum level for Ockham arrivals the old routings w closer to Ockham but even the smaller aircraft were not going to make it very close to that height -taking LHR outbounds further west means some departures could climb above the initial FL60 restriction but the further inbound aircraft are from Ockham the higher they are so that looks like no real advantage either.
So as it stands HAL have asked NATS to implement a trial which has upset a lot of people for no real gain and nor does it seem to me at least that it would improve resilience , so assuming they are not completely illogical I am left wonder WHY?
PB