you're right, that cleared it! thanks
It now shows boot loader v1.10 when reboot with the jumper off, but if i try to flash the new firmware from the USB stick by pressing Op, it hangs with a USB upgrade message (I'd previously copied the 2 bin files onto it)
alternatively, if I continue with Xavier's suggestions, I get this:
alexs-imac-70:build alexrajkovic$ . ./install_firmware_overclocked.cmd
dfu-util 0.7
Copyright 2005-2008 Weston Schmidt, Harald Welte and OpenMoko Inc.
Copyright 2010-2012 Tormod Volden and Stefan Schmidt
This program is Free Software and has ABSOLUTELY NO WARRANTY
Please report bugs to dfu-util@lists.gnumonks.org
Filter on vendor = 0x0483 product = 0xdf11
No DFU capable USB device found