Now in some respects I am very pleased with my progress, but in other respects quite distressed and appealing for help!
I have finished the tender, with reservations – it doesn’t run as well as it should and needs to!

As you can see, it looks OK (given that it isn’t painted).
However even on my S-shaped test track it de-rails: there is a compensation mechanism which should allow there to be free movement of the rear four wheels almost independent of each other but this doesn’t seem to work completely effectively. If it did, the six wheels would all be always in contact with the rail. However the front axle seem to be able to have one rim off the rail, and so with the small S7 flanges, it de-rails. Adding weight to the front end of the tender makes no difference.
I had been so careful …. !
At every stage of construction I had run the tender chassis up and down behind the locomotive and it ran without problems. However at the some stage it becomes necessary to put it all together, take a deep breath and solder it up in such a way that it cannot be taken apart to adjust it.

By the time I had the water-scoop parts and the split-axle pick-up components all crowded together, it looks like this:

Click to enlarge

I dread having to unsolder it and take it all apart again!
It looks so good.

However looks aren’t everything, and unless additional lubrication of the moving parts frees up the mechanism and allows it to run over uneven track, what else can I do?