Lots more progress to report. With a clear weekend I had plenty of time to complete the last of the preparation, buy the wood and get on with some real live construction.
First, I had to sort out the flooring. The loft had a bit of old carpet covering the walkway; when I rolled it back I found that, as well as covering the walkway, it also covered a multitude of sins...
Fortunately there was enough good chip and plywood, and some serviceable floor slats, amongst the jumble to allow me to lay a decent floor around the layout area. A permanent chip floor can wait, and it will have to: what these photos don't show is the unsightly mountain of boxes etc behind the camera! I'll need to tackle those before I can do a decent permanent job with the floor. But the result for now doesn't look too bad.
Having done all that, I was finally ready to start making the frame. A trip to Wickes ensued, wood was purchased and loaded into/onto the car (thank heavens for the roof-rack), and brought home, hoping that my knots would hold. My trip would of course have to coincide with the biggest rain-shower of the day! I was very glad yesterday not to be living in Morpeth. Anyway, the wood quickly dried and is now all safely up in the loft.
I have bought enough large planks (47x75mm cross-section) to construct a basic primary frame, supported along the sides by the main roofing timbers, along the back by the wall, and propped in the middle by legs. It's a lot of wood! Once the primary frame is built I'll make secondary frames to carry the baseboards (this will necessitate another trip to Wickes). These secondaries will be of smaller cross-section (22x47mm) and will fix directly onto the primary frame using bolts.
As can be seen in the above photo, the first transverse beam of the primary frame has been attached to the main cross-beam in the loft. Next to go in will be the transverse beam along the back wall. After that I'll put in the four longitudinal beams; that way the two remaining transverses will have something to rest on. The main challenge is, of course, getting everything level so that the baseboards don't have any underlying gradients.
It will probably all take a long time because I can't do noisy work (masonry drilling, cutting) in the evenings - our daughter sleeps in a room just below the loft hatch, and even she would surely not sleep through such a soundscape. I'll do what I can down in the garage, but some things I can only do in situ. Ah well - there's no rush. But now that I've started in earnest, I can't wait to see the first trains running.