I used Kdenlive to make a new Numbas demo video.
I was pleased with how easy it was.
Wrote a script, using the old video and the text from the Numbas front page.
Did some screen recordings to cover each of the bits.
Made 1920×1080 images using the drawings from the front page.
Recorded the script in gnome-sound-recorder.
Dragged all the files into kdenlive.
Cut out just the bits I wanted from the videos and the sound by making zones: play a clip, press I to set the start point, and O to set the end point, then Ctrl-I to save that as a zone.
Eventually realised V inserts the selected zone into the timeline.
Dragging things around on the timeline is easy. The move tool with M shifts a contiguous block of things around to make space.
Worked out that you can trim bits by dragging their sides in the timeline, and add fades or wipes by clicking the dots that appear in the corners of a clip.