Since my previous post, construction has begun and proceeded at a reasonable pace (for me). The main baseboard framework is complete, along with its supporting legs. Construction is from ply (a mixture of 6mm and 4mm). I was lucky enough to find a timber merchant who would not only supply the required 8"x4" sheets but also cut them into a complicated jigsaw of pieces (for a small fee).
As you can see from the photos, there is a simple plywood box with horizontal supports for the trackbed. The cut-outs in the sides and ends are simply to reduce the weight: a curved backscene (probably cardboard?) will eventually go in front. The top photo has some (too modern) stock to give a sense of scale. Legs were inspired by Chris Yates' article in MRJ 209, albeit using softwood rather than metal: the baseboard just rests on top and is prevented from sliding off by raised lips on the legs.
The second photo was taken today and shows progress this weekend on the fascia/lighting pelmet. This supports a cheap IKEA fluorescent tube.
The legs are still waiting for adjustable feet to be fitted to the bottom ends, after which I want to start on the two fiddle yard boards. (Unfortunately this will be one of those layouts where there is more length of fiddle yard than scenery, but I didn't want to be over-ambitious with the amount of scenery to finish.)