@Chrisedge-

Right on!

There are several sets of instructions for working with Requiem, but the BEST that I found (complete with a “How to Troubleshoot Requiem Errors?”) is this one…

-> http://www.ismoothblog.com/2012/12/apple-ibooks-and-itunes-drm-removal.html

I would suggest visiting the above link and very carefully going step-by-step through their instructions for iBook DRM removal using Requiem, but here's a summary version of what you'll want to:

1) downgrade your iTunes to the version 10.5.3 appropriate to your system (links provided)…

Uninstall your current version of iTunes (if applicable).
Set-up the necessary version of iTunes (including authorizing your computer and setting-up your iOS device).
IF already purchased in the iTunes Store, then delete and re-download the file(s) you want to decrypt.

2) Regardless of wether your PC is running a 64-bit version of Windows, install a 32-bit version of Java (link provided).

3) Download and run Requiem 3.3.6 (link provided).

Hope that helps (and please let us know your results)! ;)

-A.S.