Moving quickly on, the baseboard for the scenic section of the layout has been constructed. I largely used old plywood offcuts for this. Next, I printed out a scale copy of the track plan. This was stuck to 2mm MDF. I removed the surplus bits, leaving just the outline of the trackbed. I then glued this to the baseboard.
Having used odd scraps, the baseboard looks a little uncouth at the moment. All will be hidden by a facia and scenery in due course though. The important thing for the moment is that the board is flat, square and rigid.