In the end I had to take much advice from the fellow-members of “Western Thunder” – a model railway nerds website.  I dismantled the tender and much of the running gear and made sure that all of the compensation mechanism was free to move.   One suggestion had been to twist the chassis, but in the end I achieved much the same effect by introducing a spacer at one corner between the outer farm and the chassis, thus effectively applying a permanent twist to the latter.

The tender now runs:

However I realised that as it is the front wheels lifting off the track by 0.5mm, testing the tender on the S-shaped curve but with a dip in the track might not be good enough: the concavity of the track might keep the front wheels in contact when flat track or even a hump in the track might have the opposite effect and cause a derailment.

So instead I had to raise the centre of the test track and make the tender run over this.  The only realistic way to do this is to pull it behind the locomotive!  This became the first trial runs of the locomotive and completed tender:

It doesn’t run smoothly though – this is because there are electrical short-circuits created between the pony truck wheels and the front locomotive steps, and also between the guard irons on the tender (the metal posts which clear the rails of anything before the wheels run over it on the real thing).  However the assembly never de-rails, which is a triumph!

Here is a picture of the whole assembly as it is now.

Click to enlarge