Add text to Existing PDF using Python

I need to add some extra text to an existing PDF using Python, what is the best way to go about this and what extra modules will I need to install.

Note: Ideally I would like to be able to run this on both Windows and Linux, but at a push Linux only will do.

Thanks in advance.
Richard.

Edit: pyPDF and ReportLab look good but neither one will allow me to edit an existing PDF, are there any other options?

Add text to Existing PDF using Python