From Evernote:

This train ....

... won't pass between tracks if they're even fractionally uneven (and I mean fractionally - the drive wheel bogie needs the four wheels in each corner to be absolutely flat or it doesn't get traction) but, more annoyingly, it lists to port.  I have had the whole thing to pieces using Hornby's original data sheet but not even the mearist hint of why it should be doing this.  Ideas welcome.