Thanks for the reply.
Reading through the manual, reading data via GPIB seems quite straight-forward: DO 'foo' should request the data from channel foo. To get started, try doing the setup through the front panel and only execute the DO command through GPIB. Or try the sample programs. Can you elaborate how you think the manual is lacking? Seems like a fairly standard reference manual that assumes you are already familiar with operation of the device and explains how to map each specific action to a GPIB command.
I did try to set everything up via the front panel and then run DO command, however the front panel is disabled if you have GPIB connected and once you open the communication link the instrument resets even when *RST has not been sent (this was in Matlab)
Because of having to configure the entire setup over GPIB I quickly gave up and promptly blamed it on the manual when it was more likely my lack of understanding that was the problem.
I have now got the data off of the equipment.
For future reference, there is some software called Win4145 / WinICS which requires a license however they offer a free 30 day trial. This software is designed to communicate with the 4145A/B (and various other SMU's) and it did the job. However, it only works on PC's/laptops that are running Windows 7 or older (XP, 2000 etc)
Hope this may help someone in the future