YES - at last, finally managed to get the loose chain refitted. Took a further 15-20 minutes - but as soon as it was in the hole out came the glue. No way is that going to shift.
Now that's the good news - on re-righting the loco something fell out from the bogie - looks like a brake pad. I'm guessing that to refit will require the removal of the bogie. Looking at the instructions for this on the A/S support page, the first thing to do is remove the chains - not a chance! I'll have to live without a brake on one of the wheels.