OK. I've sorted it. I traced the issue to the original throttle linkage flexing. It runs from the throttle pedal to the nearside chassis rail and is only supported at the ends where the arms are. The new arm sits next to the offside chassis rail, the rod bends slightly preventing full throttle.
I have cut the rod in half as I don't need it to go to the nearside, the loose rod will be supported on a bracket with a bronze bush in it. It doesn't need a retaining pin as there is a split pin at the other end.