This is a modification that was suggested by Derek McLarney, with the aim of making the front end more responsive.  It definitely had the desired effect; in fact, if you do this mod before you do anything at the back the little beast will probably try to write its name down every straight.

This mod actually takes advantage of the fact that the Mardave plastic is so compliant.  What you need to do is:-

1   Hold the bottom of the "castor block" in a vice.
2   Place the kingpin in position.
3   Hold the top of the "castor block" in a pair of piers or mole grip and gently twist it through about 30 degrees whilst applying heat to the top of vertical leg with a heat gun or hair drier.

You may have to mess about with a drill to ease the holes in order to achieve the right sort of free running fit but having done all that, you should find that by twisting the top through about 30 degrees you have increased the castor by about ten degrees.
The heat softens the plastic, allowing you to position it as you wish.  When it cools it will retain its new shape.

The twist in the vertical section can be seen on the shot above.