Steve, as the nut screws on to the top of the stem and you say you have removed the stem and bottom yoke, does that mean you have removed the eccentric adjuster as well. If the stem is still in the bottom yoke you should just need a goon way of clamping that (work-mate or big vice) and then using the tool to undo the nut
It looks like the steering stem nut has not been removed for some time and it will not undo. Like you say now the lot is away from the frame will make it easyier to hopefully undo the nut. The haynes tool is good for tightening, but I am worried it will distroy the holes as they are made of alloy. Hope to have a proper tool in a few days. Its only taken a day and half to get this far