Client causing massive delay in project

Hey guys,
really need some advice in dealing with this small issue I have.

I started on a web re-design for a client of mine in late November. This project should have taken at the most (with the end of year holidays etc) until maybe mid january.
We are now in April, and they are still sending thru wording changes, still haven't provided me with the copy for some of the pages etc.
Anyhow, I have pointed out to them that the project should have been completed months ago. They have even admitted that it's been their fault because they haven't been reverting back to me quickly enough etc etc.

Cut a long story short - there have been "projects within the project".. For example flash banners, additional coding work that was out of scope, bla bla bla.

So, I want to invoice them for these additionals NOW, to basically say to them that enough dragging of your feet, get this sh*t paid now!

I'm kind of at a loss for how to let them know about this..

I mean, with all my heart I want to say to them that they have been dragging their feet and they need to pay this money to me now, but I need a NICE way of saying it.. I have a lot of other work that will commence as soon as this project is completed.. I will be sure to enforce those payment procedures then..

But in the meantime, any thoughts on how I can word this?

So far, my wording is as follows:

Hi Ladies, Attached, please find the invoice for the additional projects completed, within the main project. - Diagram drawings - Website Coding - Flash Banner - Additional Form Creation Normally, we would include this with the final invoice, however, this project has been substantially delayed

That's all i've got..
PLEASE HELP