LM555 timer

Our servos get their position from the width of a pulse that's repeated 50 times per second. The pulse measures 1.1mS at one end of the travel, 1.5mS to centre and 1.9mS to go the other end, these are industry standards. It's a simple matter to make a circuit that will drive the servo to a point using a LM555 integrated circuit.

This is the circuit and you can change resistances according to this chart and formula.

Add a small relay and you can easily make your servo go to 2 different positions for operating points on a railroad for example.
The IC can take any voltage up to about 30 and can easily source enough current to drive a servo.

The 555 varies in price from around 30 to 50 pence each, (plus p+p) then you need the external components and a circuit board, although you could use Veroboard, if you can find any!