To send midi to Fabfilter Saturn, I enable as a Synth in the plug in manager. But when I add to teh FX rack, it's no long in the FX Manager list (even though I have a customised list containing it), but in the Synth Manager list.
 
Is there a way to keep Synth enabled FX VSTs in the FX Menu
 
or is there another way for assigning midi to VST FX I've missed?

I do not believe there is a way to force a synth into the FX plug-in lists. Once an FX is configured as a synth, it will reside in the synth plug-in lists. Starting with X3 there are options in the standard header VST drop down to enable MIDI I/O for an FX plug-in.

Doh, thank you. Getting deja vu seeing that menu.
 
Guessed you'd answer, cheers