Since the missing plans have now been retrieved from the LTO, I'm now taking AC's role to geo-reference the, I followed her excellent instructions, which unfortunately no longer match the version of QGIS that's current on my machine (I've moved from 2.18 to 3.0), so it took me a long time to figure out exactly what to do. I'll rewrite those instructions as a separate version in the same documentation file for version 3.0 when I've done a couple more. The main thing that took me AGES to realize was that to stop drawing a polygon and finalize it as a feature, you have to right-click. There's no GUI control at all, and no other way to do it as far as I can see.