2000 Headway Calcs and Layout

[interesting_signal]

The question I have though is:
how do you envisage that the freight that runs from A to B is to operate?

I'm still digesting some of the comments, but I'll answer the last question first.

For the freight train my intention was that the train from A comes onto the Down Main and leaves the wagons here, behind signal 123 (at night so ok to leave on the main line/saves the energy wasted in having to haul them all the way to the station only to come back again). The loco continues to station D, and then up to 1003 LOS, reverses and continues on the main line via 134 all the way up to 120 so that it's behind 1001. From 1001 shunts up to the wagons/behind 122. Then from 122 goes back onto the Up main and continues to B.