The result is Theorem 5.2 of this article with Alexander Isaev

Handwritten notes and remarks on the talk

A sample of Clifford's handwritten notes on the invariant theory of binary quantics

My incomplete notes on Sylvester Canonical Form

An annotated Maple file (needs updating: Maple 6 I'm afraid) for calculating the invariants of a binary sextic

An annotated Maple file (needs updating: Maple 6 I'm afraid) for calculating the invariants of a binary quartic (and hence the j-invariant of an elliptic curve)