Hi Graham,
go to "Function ---> Utility ---> Config 1 ---> 3 Parameter Lock", tick the checkbox "Split Point" there and press "Exit" to save these settings. Now, Split Point Settings can only be changed directly in the Split Point display ("Direct Access ---> Sync Start"), but no more by Registrations or MIDI files.
In the same way, you can lock the "Fingering", to avoid unwanted changes by Registrations. (To change the Fingering manually press "Direct Access ---> ACMP".)
P.S.
I recommend to leave also the "Master EQ" box checked in "Parameter Lock".
Best regards,
Chris