Unable to set Alt+Print as screenshot keyboard shortcut (laptopt keyboard)

On 13.04, under System Settings -> Keyboard -> Shortcuts -> Screenshots -> Take a screenshot of a window, I am unable to set Alt + Print as the shortcut. Pressing that combination causes Mod2 + Alt L to be (incorrectly) set.

NB I am able to set 'Print' as the shortcut key for Take full screenshot, it is only in combination with the Alt key that I am unable to set the shortcut. Also, this is on a laptop keyboard (HP G61 laptop), so Print Screen is accessed by 'Fn' key + Home key, so the actual key combination I want to use is 'Fn' + Alt + Home.

My scan codes for the keys, if it helps (sudo /lib/udev/keymap -i input/event4 > ~/keymap.log):

Alt L:
scan code: 0x38 key code: leftalt

Home:
scan code: 0xC7 key code: home

'Fn' + Home:
scan code: 0xB7 key code: sysrq

'Fn' + Alt L + Home
scan code: 0x38 key code: leftalt
scan code: 0x54 key code: sysrq

Ah, just to complicate matters further, 'Fn' + End also gives: scan code: 0x54 key code: sysrq

Further info - I am trying to reset the shortcuts back to the defaults, i.e. they did originally work with Print = full screenshot and Alt Print = window screenshot, I was messing around with trying to use Shutter instead, decided I didn't like it, and now am trying to set the shortcuts back to the defaults.

Further further info, I have a sort of workaround by using CCSM keybindings for Alt+Print bound to gnome-screenshot --window, which I feel is less than ideal (although it at least does what I want!).