  Given a parametrised weight function $\omega(x,\mu)$ such that the quotients
of its consecutive moments are M\"obius maps, it is possible to express the
underlying biorthogonal polynomials in a closed form \cite{IN2}. In the present
paper we address ourselves to two related issues. Firstly, we demonstrate that,
subject to additional assumptions, every such $\omega$ obeys (in $x$) a linear
differential equation whose solution is a generalized hypergeometric function.
Secondly, using a generalization of standard divided differences, we present a
new explicit representation of the underlying orthogonal polynomials.
