Hi thanks for the replies, there actually very simple, the main body is simply turned, then the position of the step was marked onto the bars. This enabled me to drill 3 holes each side (to hold as well as centre the plug in the bar), these were then tapped to accept a 3mm grub screw (ground at one end to enable them to sit flush with the bar once tightened)

Im not usually quite so obsessive about my bikes, although Ive only recently been trained on a lathe and didnt really want to fork out for plugs which may just get ripped out in a few months..