Following from this post Servo Tester Modified, found a design for Servo Controller based on the NE555 Timer IC.
The original idea has been kicking around and languishing in a A4 Box File for at least 22 years!
Another one sorted from the Round Tuit list!
Took several attempts, and several component changes, to produce a reliable working example.
Updated and uprated the components.
Transistor changed from the original one in the design.
Eventually settled on the Presets, better overall control for the Servo movement.
Additional capacitor, and changed all the resistors to 1% tolerant type.
Does require a stable and reliable 5V regulated power supply.
Operation to activate, same as the Servo Modified Tester, switch or Optocoupler to make the connection.