Major good news! After the dispiriting news of jamming gears, I put it away for a day or two while I got on with something else. After carefully removing the wheels and gears until it freed up, I traced the problem, I think, to one of the short stub axles on the gear train, it was just too short, so I replaced it.
After that it all ran smoothly, so I carried on, reassembled everything, even the rods, and it still ran! So I soldered the rods on with the microscopic washers and oil soake