Attached a sketch of a bit of circuit to do the job. Since Maplin have decided to hide the useful stuff, even if they still sell it, I looked elsewhere found the parts in ESR's on line catalogue (
http://www.esr.co.uk/ ).
It consists of a device to watch the output from the ESC (the opto coupler), which them mimics the pulses to the output transistor. This drives what I have called the fan motor. The resistor paralleling the transistor provides a bleed current to let that motor turn at you preferred minimum speed.
Pules from the ESC switch one of the diodes on and off. The light from that diode switches its corresponding opto transistor on and off. Forward does one pair, reverse does the other. Either pulses the output transistor, which drives the motor faster or slower, but always in the same direction.
Using an opto isolator saves a lot of thinking in the design department, the Darlington transistor was chosen because a/ I am used to it, b/ its fairly bullet proof c/ it needs very little drive current d/ fairly cheap. The idle bleed resistor value depends on the motor and supply voltage, but a ceramic type of fairly high wattage would be the best type. Almost any variable type would just add to the smoke before dying. With that resistor, there should be no need for a protection diode, but the motor should have normal suppression fitted.