Thanks to some very clever design work by Terrytoolpath, he came up with a blade holder that attaches to a standard 5mm handle and is sufficiently wide to get the correct bevel angle - I'll take some pictures at a later stage to show how it works.
So this morning I took a Feather pro blade that had been used twice and honed it on a Japanese natural stone then stropped on leather and sealskin ready for tonight's shave.
So how did it go in my Feather straight? I have to say that it added nothing and, in fact, made the blade a bit tuggy - after the first pass I finished off on a standard straight. I'm pretty sure that Terry's honing attachment was not at fault, I've come to the conclusion that these blades just won't be improved on a hone. Maybe they are coated and the hone removes it?
Anyway, thanks very much to Terry for his help.