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 soaked rizla paper. 
 

It still runs. So far so wonderful. Stay tuned while I put the cosmetic outer frames on and so on.

Once I’ve recovered from the celebration!