Before I can do anything, I've decided that I'm going to need a new, slightly bigger shed. I'm hoping for an 8'x6', with the door on the end, as normal. Around 3 sides will be a worksurface/baseboard at 1m high (lower than the current railway), with shelves at sensible heights above and below. The railway will be run on the back and right hand walls as you walk in, in a similar configuration to the current shed. The left hand worksurface will become a general use work bench.
As for the railway itself, I'm going to build it in smaller sections, aiming each section to have a good amount of scenery and detail, and will be protected from the outside elements by a hard cover. This won't prevent all the damage by the conditions but should stop most. The scenic areas will be connected together by very bland railway lines, which will have little in the way of features that can be damaged by the weather, with the connection between the two being a tunnel portal or similar.
The scenic areas will no longer be flat boards, There will be a lower frame of thick ply, which will carry a very thin layer of ply on supports to carry the track. This will make it simpler to vary the height of certain areas of the layout.
Here is a picture of the first scenic board, in the planning stage...
