enforcing a consent order-how?

Been divorced 9 years, got a consent order in place which includes child support payments, the amount has been changed via a hearing around 5 years ago to a lower amount. My ex now pays when he feels like and has halved the lower amount, this month I have had nothing at all so need to enforce the order - does anyone know what form and what I will need in preparation to do this in person?
I have done all of my own legal work in the past but can't seem to find info on this one! It is a pre 2003 order so can't use CSA.

I have since found that the D11 form seems to be the right form and it was easy enough to fill in. Not sure if I use the old case ref number though or leave it blank.