So yesterday I did a little more work to the manual shaper to reduce some of the play in the cross slide.
I removed the guide spacers and measured them against the cross slide rail. The rail measure 10.9mm while the guide spacers measured 11.9mm. So my movement was coming from the 1mm of clearance between the two parts.
I decided to machine 0.75mm off the guide spacers to create a 0.25mm clearance between the cross slide rail and the saddle / guides.
This made a massive improvement without completely locking up the saddle. Let me know what clearance you would have made?