Thanks Steve. I've checked both Hornby-chassis locos and they sit nice and square. The Hawthorn Leslie has an unmodified chassis but even with the extra weight, still surges. There have been some interesting answers on the Greenwich site and Richard Glover came up with what may be at the root of the problem. This is what he said:
I think the fact that the old Dock Shunter, which certainly has a chunky, iron cored, open framed motor, runs happily is the clue. Even cheap modern chassis usually use nice can motors, which are efficient and have low current consumption.
"Feedback controllers like the Gaugemaster and the KPC were designed for relatively in-efficient, open framed motors and usually did a good job of controlling them. But higher efficiency cans produce more back emf for a given speed and this can lead to poor control and surging. I’ve used KPCs for many years and originally they gave good results. The originals finally wore out, but the replacements were rather disappointing. I came to the conclusion that the problem was changes in the types of motor in the latest generation of chassis.
The old style resistance controller is no help either, as they were designed for motors that drew an amp or more and will give little or no speed control with modern motors.
One answer is a true, variable voltage controller. The non-feedback Gaugemaster is of this type as is their Type W handheld."