Thank you for all your comments and recommendations. Including the time David has taken to show recommendations in a doodle.
I intend to use DCC. The colours were only to split into sections, not to indicate wiring design. Thought it would help you make your comments.
Coming back on the comments.
ACCESS
I have the advantage of space, so will be able to walk around all sides.
I have attached a better screenshot of the top board, it will be open, will only have maybe one tunnel port etc. I am thinking that the back edge could fold down to allow access. See photo.
So access to the hidden point, rerailers and prodding should be possible.
That being said, I do take on board comments about access being difficult. I have not counted out the option of a centre access hole. Either permanent or a scenic'ed 'drain cover'. I aim to build the board supports and wiring with this in mind.
LOWER SHUNTING AREA
I can see the logic of moving this to allow lengthening. I will redesign to allow this.
GRADIENT
See Gradient screenshot attached. I aim the top board to be 9mm ply. The upper board track height is 90mm. Which leaves about 80mm 'head height' underneath. Does this makes sense? 80mm seems just OK for access. Bearing in mind I was trying to keep the top board low to allow gradient as shallow as possible.
MAIN LINE CROSSOVERS
I will remove the right edge crossover, include a crossover at top as in doodle. Swapping crossover position will allow for the gradient to remain at 1:30 (crossover points will be flat, not inclined).
As ever, I welcome your thoughts.