By the way, this is my planned bike:
http://www.marin.co.uk/2011/bikepage.php?ModNo=11M29

At the bottom with the frame measurements it says 27.2 for the seatpost height. So this means that assuming I'm not raising the height of the saddle, I can fit a 300mm D lock through the rails to the toptube with nearly 3cm to spare?

Assuming I am right about what they mean by seatpost height then hopefully with the extra 3cm, maybe I can even Fit the D Lock onto the tube just behind the seatpost. The thing which this person has attached the chain to:
http://www.flickr.com/photos/7612786...2842/lightbox/

Still interested to hear about Slim D Lock vs Slim Chain and padlock

Otherwise, I'll report back on my own findings when searching tomorrow.