Had a look round to try and find a front end filter that wont trash the input impedance.  Cant find much, but I posted a few posts on forums, so we shall see what turns up there:

here, here and here.

Phil

According to Feynmens lectures on physics (pp. 8-12), the electrostatic equations are not valid for close proximity electrtostatic forces.  This leaves the equation field wide open to essentially use what I want.  Playing with Excel found that the best (where simple) equation to use was 1/(1+2.7*(x^3)).

The graphed output of this equation looks like: