Wow - thanks for all the great input guys!
The 70-200 options sure seems to be getting votes here, so I am now considering it, although it would certainly stretch my budget to it's max. I might help if I give a few more details on my current gear and the things I like to shoot:
-Canon 17-40 L
-Canon 50 1.8ii
-Remote shutter release
-Canon 430 exii for on-the-go lighting
-Off-camera flashgun lead for 430exii
- a couple of old Vivitar 285 strobes for off camera flash work (long exposure light painting etc...)
-Interfit EXD200 studio strobes (x2)
-Infrared trigger for strobes
So... I do have the 430exii which I use even in conjunction with the 50mm (as well as the 17-40 L) indoors for added light or when I don't want the shallow DOF of 1.8. The other thing I will share here is my flickr link so that you guys might get a bit of a better understanding of my needs/wants... I feel a little shy about giving the link on here, as most of it I don't think can be qualified as 'real photography'. Don't get me wrong, I enjoy what I do, but it's not always to everyone's taste as I do go a little ott with the ol' processing...
Essentially, I am not a photographer - it's something that I really enjoy doing and it does play into my workflow as a graphic designer, who spent a good few years working on album sleeves, so Photoshop has always been a big part of shooting and image editing for me.
Anyways, you can have a look over here:http://www.flickr.com/photos/cowgummy/
Thanks again so very much!