I spent more time looking at the desktop monitor thing, and realised that the 2511x (driven by the HDMI port) was scaled down - I couldn't get the Catalyst Control Centre to see the monitors, so I pulled apart the office, and set it all up again. The monitors are now driven by VGA and DVI, they just swap between the laptop and the desktop. I've also put the HP on the left, so it doesn't reflect the back door - it has a shiny screen :o)
I upgraded the graphics drivers and gained control via the CCC, but also lost the scanning line, so imma call that good for the moment :o)
I can't switch between the laptop and the desktop without playing with the monitor inputs, but I pretty much never run them both at the same time anyway.