A bit more thinking time was all that was needed in the end (as is usually the case). The result looks a little like an earthwork plan of a hillfort entrance at present, but tracklaying will change that, I hope.
To give a sense of what I'm trying to do, imagine that the mainline railway is set along the contour of a very gentle hill on the edge of low lying land. The quarry line is coming across that low lying land and has to gain a relatively small amount of height to meet it but doesn't have the advantage of an act of parliament to persuade the farm to part with their dairy pasture. This explains both the occupation bridge that will hide its exit and the odd switchback required for the industrial line to have a loop as part of its exchange siding. And who doesn't like a short length of 1 in 30?
Operationally, this also means that the loaded wagons from the quarry have to be brought into the flat part of the loop, run round, possibly split and pushed over the hump. Empties will be lifted over the gentle(!) side which comes in at about 1 in 50 before disappearing offscene. This no doubt accounts for the rapid attrition rate of industrial locos on that system... In this, I'm informed by Iain Rice's suggestion that some deliberate awkwardness makes for more interesting operation (exhibitions) or play value (at home).
Now look at the trackplan:
The 'mainline', meanwhile, heads off towards Ilchester at 1 in 100, perched up on a low embankment leaving the level goods siding behind. Practically, this also prevents runaways in the layout's eventual home down at the bottom of the garden - 'Ilchester' will only exist when the sun shines and I can have the door open but its mousehole will always be present.
I probably ought to think about track at this point. I wonder how much I have in hand?
Adam