@ian-tech

As you only have one heater, try putting this in your start gcode

G10 P0 S205 R205
G10 P1 S205 R205 (or whatever temperature you want to print at). That'll set the active and standby temperatures the same so you can instantly switch between tools.

Then make sure that Cura does nothing other than select a tool by putting in a "Tn" command (where "n" will be either 0 or 1). Oh, and make sure that there is nothing in your tool change files - at least for now.