Thank you all.
I keep Layer 0 on all the time - in fact I am using it, but I had no idea it held anything other what I had deliberately put there: the centre-lines and dimensions.
I'm not trying to be "puritanical" in restricting myself to just 2 layers - I know they have their uses but I am trying to keep things simple.
I know the 2D/3D Selector. I used it to see if somehow I had placed something somewhere "above" the rest of the drawing. I hadn't, which made this work-plane business all the more baffling. I tried that "Select all > Place on Workplane" but it didn't re-set the troublesome parts.
I'm not sure what I hoped to see, but I turned on "Show Workplane". It did - as a small square over part of one view, so not very informative. I turned it off again.
I am not using any but the default file-type, as I am not importing drawings from other software. I'll leave that to the experts!
I considered drawing only on the Paper Space screen but I thought I would not be able to work as I do: a series of main and derived views.
If the Model Space so far for this engine, which is less than a foot long, was drawn in real size it would be some 6 feet long. That's because I made the General Arrangement's front and end elevations at the left-hand end, then construct across to draw the individual parts; with key centre-lines like the crankshaft's, and the overall heights, all consistent.
I had drawn the crankshaft on its own. In elevation it's a chain of rectangles but I then copied it down the same page to group it, for copying to the main drawings. I was able to "shadow it" (thin green, dashed outline) but found I had to do that on its original, and then only by exploding it first. That worked, but copies of other parts I had drawn and grouped on one view though, took up their new location's line properties when I placed them on another.
How do I know it happens|? It's what I see on the screen!
Since all these things seemed to be happening together I assumes the effects were somehow linked.
"If you want to just about guarantee a 2D world ..." Not so much "want" as having no choice, Mike, but for this specific project, a set of orthographic views gives me information an isometric one won't.
I had hoped to be able to use TC for isometric drawings, and I have created a few simple exercises from trial-and-error to the one of this engine's cross-heads. Even though the cross-head's 3D drawing is of a real thing, it has no further use, and beyond this level is impossible.