Cheers! What about this one here, also from the 1976 spec (italics
3.1.6 Double Height
A receiver responding to one or more occurences of the Double Height mode in Row 'R' will operate as otherwise during that Row except that in every Character Rectangle during the Double Height mode only the upper half of what would otherwise have been displayed is displayed, stretched vertically to fill the rectangle. On Row 'R+1' the corresponding lower half of each of these is similarly displayed. The remaining Character Rectangles of Row 'R+1' are to be displayed as unboxed spaces of the same Background colour as the corresponding Character Rectangles of Row 'R'.
To me this implies double-height backgrounds only get copied down after Double Height 0x8D, and stop getting copied down after Normal Height 0x8C. But neither edit.tf, zxnet nor Richard's telstar.bbc do that - they copy everything down if there's at least one Double Height 0x8D.
At the moment my own parser doesn't agree with this test page
or anybody else, which is what flagged this up (as I was coincidentally following my own interpretation of that paragraph, before I reread it closely!) DH Doesn't get set till column 2, and gets applied in column 3: