For anyone else wanting to do this on a GDB4, the GDB3 Guide and binaries work ok, with the following changes, because the GDB4 doesn't have a serial port or a reset button.
Please note that the newer GDB3 with no RS232 WILL NOT work with the 3.x or 4.x version software. No attempts must be made to try and update from 1.8 to 3.x or 4.x as the box will no longer work.
1) You need a make a 9 pin Serial to Scart cable (yes, really, they've used some unused scart pins for a serial port ) with 3 wires connected to the pins as follows:
See Appendix B of GDB4 manual (http://www.goodmansdigital.co.uk/In..._Books/GDB4.pdf ) if you don't believe me
2) Where the guide says press the reset button, unplug the dc supply and plug in it again.
Thanks to PhilipB from digitalspy and all who've helped.